2025 IB DP Computer Science 模擬試題連答案詳解

Under direct changeover, the old system is completely shut down and replaced by the new system on a specific date. If the new system has critical, undetected bugs or fails entirely, there is no redundant system operating simultaneously (unlike in a parallel changeover). Consequently, the organization has no immediate fall-back option, leading to severe operational downtime or permanent loss of data.

[1 mark] For explaining the mechanism of direct changeover (immediate cutoff with no overlap).
[1 mark] For identifying the consequence of failure (no backup/fall-back system available).
[0.5 marks] For linking this consequence to business disruption, operational downtime, or data loss.

Elderly users often experience physical limitations such as declining visual acuity or reduced motor control (hand tremors). Two usability factors to address are text size/readability and target size (buttons). Text size can be addressed by designing a highly visible font with adjustable text scale settings and a high-contrast mode, allowing users with visual impairments to easily read the interface.

[1 mark] For identifying two appropriate usability factors (e.g., font size, contrast, target click area size, simplicity of navigation layout).
[1 mark] For explaining a concrete interface design choice that addresses one of these factors.
[0.5 marks] For linking the design choice to the specific physical or cognitive limitation of elderly users (e.g., tremors, poor eyesight).

At the beginning of the fetch stage, the Program Counter (PC) holds the memory address of the next instruction to be executed. This address is sent via the address bus to the Memory Address Register (MAR). Immediately after this address is copied, the Program Counter is incremented by 1 (or by the size of the instruction) so that it points to the memory address of the subsequent instruction for the next cycle.

[1 mark] For stating that the Program Counter stores the address of the next instruction to be fetched.
[1 mark] For stating that this address is copied/sent to the Memory Address Register (MAR).
[0.5 marks] For stating that the Program Counter is incremented at the end of this phase to point to the next instruction.

In packet-switching networks, a data stream is divided into individual packets. Each packet is treated as an independent unit and routed dynamically. Routers determine the path for each packet based on current network state. Because of dynamic routing, packets can take different physical routes with varying lengths, hop counts, and traffic congestion. Some packets will experience longer latency than others, causing later-sent packets to arrive before earlier-sent packets.

[1 mark] For explaining that packets are routed independently of one another.
[1 mark] For explaining that packets can take different physical paths/routes.
[0.5 marks] For explaining that differences in latency, path distance, or network congestion cause varying arrival times.

WPA2 encrypts the actual data payload traveling over the wireless waves using strong encryption algorithms (like AES). This means even if an attacker intercepts the radio signals, they cannot decrypt the data. In contrast, MAC address filtering does not encrypt data; it simply checks the hardware address of the connecting device against an allowed list. Since MAC addresses are sent in plain text, an attacker can sniff a legitimate address and easily spoof it to gain access to the network.

[1 mark] For describing how WPA2 provides data confidentiality through encryption.
[1 mark] For describing how MAC filtering operates purely on hardware identification and lacks encryption.
[0.5 marks] For contrasting their security levels (WPA2 is cryptographically secure, whereas MAC addresses are sent in plain text and easily spoofed).

Decomposition is the process of breaking a complex problem or system into smaller, more manageable, and distinct parts. A key benefit of decomposition in software development is modularity: it allows multiple programmers to work on different components of the software in parallel, makes the code easier to test and debug, and increases the reusability of individual modules.

[1 mark] For providing an accurate definition of decomposition (breaking a complex problem/system into smaller sub-units).
[1 mark] For stating a valid development benefit (e.g., parallel development, modularity, easier debugging/testing, code reusability).
[0.5 marks] For explaining how that benefit is achieved (e.g., by isolating tasks so different teams can focus on specific sub-components independently).

In binary search, the worst-case scenario occurs when the search space is repeatedly halved until a single element remains. For an array size of \(N = 128\), the maximum number of steps/comparisons is calculated as:
\(\log_2(128) = 7\) comparison divisions.
In the worst case (unsuccessful search or finding the element on the final division), it requires at most \(\lfloor \log_2(128) \rfloor + 1 = 8\) comparisons.

[1 mark] For stating the correct answer of 8 (or 7, depending on whether the final step evaluates to 1 element directly without an extra check; accept either 7 or 8 with matching logical calculation).
[1 mark] For showing the logarithmic relationship or the step-by-step halving of the array size (128 -> 64 -> 32 -> 16 -> 8 -> 4 -> 2 -> 1).
[0.5 marks] For explaining that the binary search works by splitting the sorted array in half at each iteration.

Inheritance allows a subclass (child class) to inherit attributes and methods from a superclass (parent class), establishing an "is-a" relationship. Polymorphism builds upon this relationship: it allows the program to process objects of different subclasses through a single common interface of the superclass. At runtime, the computer executes the specific overridden version of the method belonging to the subclass of the actual object, enabling dynamic binding.

[1 mark] For explaining that inheritance establishes a class hierarchy where child classes inherit state and behavior from a parent class.
[1 mark] For explaining that polymorphism allows a single interface to represent different underlying forms (overriding inherited methods).
[0.5 marks] For explaining the connection (polymorphism requires inheritance or interfaces to dynamically resolve overridden behaviors at runtime).

Usability is a measure of the overall user experience, focusing on factors like learnability, efficiency of use, memorability, error prevention, and user satisfaction. Accessibility, on the other hand, is a subset of usability that specifically addresses making the software inclusive and usable for people with disabilities (such as visual, auditory, cognitive, or physical impairments), often requiring compatibility with assistive technologies like screen readers.

Award 1 mark for explaining usability (e.g., focusing on user efficiency, satisfaction, or general ease of use). Award 1 mark for explaining accessibility (e.g., focusing on inclusivity, removing barriers, or designing for users with diverse physical or cognitive impairments). Award 0.5 marks for a clear comparison or distinction that demonstrates how they relate (e.g., accessibility is a specific dimension of usability targeted at avoiding exclusion).

1. Calculations: A 12-bit address bus can address a maximum of \(2^{12}\) unique memory addresses, which equals 4096 locations. 2. Role of the MAR: During the fetch stage, the address stored in the Program Counter (PC) is copied to the MAR. The MAR then places this address on the address bus to point to the specific memory location in RAM from which the next instruction needs to be retrieved.

Award 1 mark for the correct calculation of \(2^{12}\) or 4096 memory locations. Award 1 mark for stating that the MAR holds the address of the data or instruction currently being read from or written to RAM. Award 0.5 marks for describing its specific role during the fetch stage (e.g., receiving the address from the Program Counter and placing it onto the physical address bus).

(a) Data migration issues include:
1. Incompatible data formats between the old system databases and the new database schema (e.g., mismatching fields, card sequence limits).
2. Loss of balance or user profile data during export/import processes due to connection issues or database validation mismatches.

(b) Beta testing exposes the contactless ticketing system to actual commuters under real-world conditions. This uncovers edge-case software/hardware bugs (such as card-reading lag at turnstiles) that laboratory testing cannot replicate. It also lets developers gather qualitative feedback on the user interfaces (sounds, screens) to refine usability before the full public release.

(c) Direct Changeover involves completely shutting off the old magnetic card system and starting the contactless system instantly.
Advantage: Low cost with no need to run dual physical components or databases concurrently.
Disadvantage: Extremely high risk; if it fails, transit operations halt.

Parallel Running operates both old and new systems simultaneously.
Advantage: Zero disruption as the old system serves as a live backup if the new system fails.
Disadvantage: Extremely costly and operationally complex to sync dual databases and physical turnstiles.

(d) Pseudocode:
method calculateFare(weeklySpent, rawFare)
if weeklySpent >= 30.0 then
return 0.0
else if (weeklySpent + rawFare) > 30.0 then
return 30.0 - weeklySpent
else
return rawFare
end if
end method

Part (a) [2 marks]:
- Award 1 mark for each valid, distinct migration issue identified up to 2 marks (e.g., incompatible formats, data loss, incomplete user files, corruption).

Part (b) [3 marks]:
- 1 mark for identifying that beta testing involves end-users (commuters) in real scenarios.
- 1 mark for explaining that it reveals real-world hardware/software edge cases.
- 1 mark for highlighting feedback on usability or operational speed.

Part (c) [4 marks]:
- 1 mark for defining direct changeover with its distinct advantage/disadvantage.
- 1 mark for defining parallel running with its distinct advantage/disadvantage.
- 2 marks for analytical comparison showing clear balance of risk vs cost within the transit context.

Part (d) [6 marks]:
- 1 mark: Correct method signature and parameter names.
- 1 mark: Valid base condition checking if weeklySpent >= 30.0 and returning 0.0.
- 2 marks: Correct condition for partial cap reach (weeklySpent + rawFare > 30.0) and calculating correct return value (30.0 - weeklySpent).
- 1 mark: Else condition returning the regular rawFare.
- 1 mark: Correct logical structures (proper nesting/sequence structure, and end statement).

(a) Benefits of WLAN in school:
1. Mobility: Students and teachers can access learning resources seamlessly anywhere on campus (classroom, library, courtyard).
2. Scalability/Cost: Easily supports hundreds of personal devices without running thousands of expensive physical cables to desks.

(b) VLAN logical partitioning benefits:
1. Security: Isolates student devices from administrative systems (containing grades, financial information, staff communications). Even if a student's device is compromised, malware cannot easily propagate to the admin subnet.
2. Traffic Management: Restricts broad broadcast domains, ensuring multicast protocols do not saturate the entire school's bandwidth. Bandwidth can be partitioned, prioritizing teacher video conferencing over student gaming traffic.

(c) DHCP DORA Process:
1. Discover: Client broadcasts a DHCP Discover packet to locate available DHCP servers.
2. Offer: DHCP servers propose an IP configuration (IP, subnet, default gateway) in a DHCP Offer packet.
3. Request: Client broadcasts a DHCP Request packet, selecting and accepting the specific offered parameters.
4. Acknowledge (ACK): Server sends a DHCP ACK packet, completing the handshake and leasing the IP address to the client.

(d) Wireless encryption details:
1. Wireless radio waves broadcast in all directions, permitting passive sniffing of packet content by anyone in geographic range.
2. WPA3 encrypts data payloads using cryptographic mathematical algorithms, transforming plaintext into ciphertext before it is broadcast.
3. Each user session utilizes unique session keys negotiated during initialization (using SAE protocol in WPA3).
4. Unauthorized interceptors without the proper mathematical key see only unreadable garbled data, maintaining user data confidentiality even on shared campus networks.

Part (a) [2 marks]:
- Award 1 mark for each valid, distinct advantage identified up to 2 marks (mobility, flexibility, reduced installation/cabling cost, support for BYOD devices).

Part (b) [4 marks]:
- 1 mark: Explaining VLAN partition logic.
- 1 mark: Connecting VLAN to security isolation (preventing unauthorized cross-access to sensitive admin data).
- 1 mark: Explaining traffic management (broadcast containment).
- 1 mark: Contextualizing to a school's actual user groups (students vs. administration).

Part (c) [4 marks]:
- 1 mark for each step of DORA correctly described:
- Discover (Client broadcast search)
- Offer (Server address configuration recommendation)
- Request (Client selection acknowledgement)
- Acknowledge/ACK (Server lease confirmation)

Part (d) [5 marks]:
- 1 mark: Highlighting wireless signals as open media vulnerable to interception/sniffing.
- 1 mark: Defining encryption as converting plaintext to ciphertext.
- 1 mark: Explaining that keys are needed for decryption.
- 1 mark: Explaining WPA3's robust session-specific protection.
- 1 mark: Stating the resulting preservation of privacy/confidentiality for sensitive client transactions.

(a) An Abstract Data Type (ADT) is a logical specification of a data structure and its associated behaviors (operations), which separates the user-facing interface from the concrete low-level implementation details in any particular programming language.

(b) Differences:
1. Standard Queue: Operates on strict First-In-First-Out (FIFO) rules. Elements are appended only at the tail (rear) and pulled strictly from the head (front).
2. Priority Queue: Elements are assigned a priority rank. Regardless of arrival time, elements with higher priority are dequeued (deleted) before lower priority items. Alternatively, insertion sorts elements immediately by priority rank.

(c) Disadvantage of array-based queue:
Arrays are static data structures, meaning they have a fixed maximum capacity declared upon initiation. This can lead to queue overflow errors if print volumes peak, or memory waste if overallocated. Furthermore, standard non-circular array queue deletions require shifting elements left, which is an inefficient O(N) operation compared to a O(1) pointer reassignment in linked lists.

(d) Pseudocode Implementation:
method getNextJob()
if not highQueue.isEmpty() then
return highQueue.dequeue()
else if not medQueue.isEmpty() then
return medQueue.dequeue()
else if not lowQueue.isEmpty() then
return lowQueue.dequeue()
else
return null
end if
end method

Part (a) [2 marks]:
- 1 mark for conceptual modeling/definition of data and methods.
- 1 mark for highlighting implementation independence (separation of interface from coding details).

Part (b) [3 marks]:
- 1 mark: Explaining standard queue FIFO behavior.
- 1 mark: Explaining priority queue priority-based deletion behavior.
- 1 mark: Contrast demonstrating how priority overrides chronological sequence.

Part (c) [2 marks]:
- 1 mark: Pointing out static size limitation (leading to overflow/waste) or shifting overhead.
- 1 mark: Explaining how dynamic linked lists avoid this via runtime node allocation and pointers.

Part (d) [8 marks]:
- 1 mark: Correct method signature declaration (method getNextJob() and matching end).
- 2 marks: Correctly checking if highQueue is not empty (using .isEmpty()) and returning .dequeue().
- 2 marks: Correctly structured fallback to medQueue if highQueue was empty.
- 2 marks: Correctly structured fallback to lowQueue if both upper tiers are empty.
- 1 mark: Correctly returning null when all conditions are exhausted, maintaining syntactic clarity.

Part (a)
- The relationship between Activity and Run is inheritance (or an "is-a" relationship). Run extends Activity, meaning Run inherits all attributes and behaviors of Activity while adding its own specific property (averagePace).
- The relationship between UserAccount and Activity is aggregation (or a "has-a" relationship). UserAccount holds references to multiple Activity objects in an array. The lifetime of Activity objects is independent of the UserAccount object (i.e., deleting a UserAccount does not necessarily destroy the activities if they are tracked elsewhere).

Part (b)
```java
public class Activity {
private String id;
private int durationMinutes;
private double caloriesBurned;

public Activity(String id, int durationMinutes, double caloriesBurned) {
this.id = id;
this.durationMinutes = durationMinutes;
this.caloriesBurned = caloriesBurned;
}

public String getId() {
return this.id;
}

public int getDurationMinutes() {
return this.durationMinutes;
}

public double getCaloriesBurned() {
return this.caloriesBurned;
}

public String calculateIntensity() {
double rate = this.caloriesBurned / this.durationMinutes;
if (rate < 5) {
return "Low";
} else if (rate <= 10) {
return "Medium";
} else {
return "High";
}
}
}
```

Part (c)
```java
public double getTotalCaloriesByIntensity(String intensity) {
double total = 0.0;
for (int i = 0; i < completedActivities.length; i++) {
if (completedActivities[i] != null) {
if (completedActivities[i].calculateIntensity().equals(intensity)) {
total += completedActivities[i].getCaloriesBurned();
}
}
}
return total;
}
```

Part (a): [4 Marks]
- [1 mark] for identifying the relationship between Activity and Run as Inheritance.
- [1 mark] for explaining that Run is a subclass of Activity and inherits its properties.
- [1 mark] for identifying the relationship between UserAccount and Activity as Aggregation / Association.
- [1 mark] for explaining that UserAccount contains/has a collection of Activity objects.

Part (b): [6 Marks]
- [1 mark] for declaring the three private instance variables with correct data types.
- [1 mark] for the correct constructor initialization of these three variables.
- [1 mark] for correct implementations of accessor (getter) methods.
- [1 mark] for calculating the correct rate of calories burned per minute: caloriesBurned / durationMinutes.
- [2 marks] for implementing correct conditional logic (if-else structure) to return "Low", "Medium", and "High" correctly based on the rate threshold.

Part (c): [5 Marks]
- [1 mark] for initializing a total counter to 0.0.
- [1 mark] for setting up a correct loop through the array length.
- [1 mark] for checking if the current element is not null before accessing its methods.
- [1 mark] for correctly matching the returned intensity string using .equals(intensity).
- [1 mark] for correctly summing the matching values and returning the final total variable.

Part (a)
- Polymorphism is the ability of different objects to respond to the same message (method call) in different ways based on their class definition.
- In this scenario, both Movie and Series inherit from MediaContent, and both implement the abstract method getLicensingFee() with different logic.
- A program can iterate through a single collection (such as an array or list) of type MediaContent, call contentItem.getLicensingFee() on each item, and Java's dynamic binding will execute the overridden subclass method depending on whether the object is actually a Movie or a Series.

Part (b)
```java
public abstract class MediaContent {
private String title;
private double basePrice;

public MediaContent(String title, double basePrice) {
this.title = title;
this.basePrice = basePrice;
}

public String getTitle() {
return this.title;
}

public double getBasePrice() {
return this.basePrice;
}

public abstract double getLicensingFee();
}
```

Part (c)
```java
public class Movie extends MediaContent {
private int duration;

public Movie(String title, double basePrice, int duration) {
super(title, basePrice);
this.duration = duration;
}

public int getDuration() {
return this.duration;
}

@Override
public double getLicensingFee() {
if (this.duration > 120) {
int extraMinutes = this.duration - 120;
return this.getBasePrice() + (extraMinutes * 0.05);
} else {
return this.getBasePrice();
}
}
}
```

Part (a): [4 Marks]
- [1 mark] for a valid definition of polymorphism (objects showing different behaviors to the same method call).
- [1 mark] for noting that dynamic binding (run-time polymorphism) resolves the method call at execution time.
- [1 mark] for explaining that a single array/collection of MediaContent can store both Movie and Series objects.
- [1 mark] for showing that calling .getLicensingFee() in a loop will call the correct specific formula for each class without explicit casting.

Part (b): [5 Marks]
- [1 mark] for declaring the class as abstract with correct syntax: public abstract class MediaContent.
- [1 mark] for defining the private instance variables with proper data types.
- [1 mark] for the correct constructor initialization of title and basePrice.
- [1 mark] for implementing appropriate public getter methods.
- [1 mark] for the correct abstract method signature: public abstract double getLicensingFee(); (with no method body).

Part (c): [6 Marks]
- [1 mark] for declaring subclass inheritance using extends MediaContent.
- [1 mark] for implementing private field duration and calling super(title, basePrice) correctly inside the constructor.
- [1 mark] for correctly implementing a getter for duration.
- [1 mark] for overriding the method signature public double getLicensingFee().
- [1 mark] for writing a conditional branch checking if duration > 120.
- [1 mark] for accurately calculating and returning the licensing fee in both conditional branches using getBasePrice() (or super.getBasePrice()).

Part (a)
- Composition implies a strong ownership relationship where the child object cannot exist independently of the parent object. If the parent object is destroyed, the child objects are also destroyed.
- Aggregation implies a weaker relationship where the child object can exist independently of the parent container.
- For StorageShelf and Box, aggregation is more appropriate because boxes are physically independent physical entities. When a shelf is deleted or dismantled, the boxes themselves are simply relocated or kept in storage elsewhere; they do not cease to exist.

Part (b)
```java
public boolean addBox(Box newBox) {
for (int i = 0; i < shelves.length; i++) {
if (shelves[i] == null) {
shelves[i] = newBox;
return true;
}
}
return false;
}
```

Part (c)
```java
public Box removeHeaviest() {
double maxWeight = -1.0;
int heaviestIndex = -1;

for (int i = 0; i < shelves.length; i++) {
if (shelves[i] != null) {
if (shelves[i].getWeight() > maxWeight) {
maxWeight = shelves[i].getWeight();
heaviestIndex = i;
}
}
}

if (heaviestIndex != -1) {
Box heaviestBox = shelves[heaviestIndex];
shelves[heaviestIndex] = null;
return heaviestBox;
}

return null;
}
```

Part (a): [4 Marks]
- [1 mark] for defining Composition (strong ownership, lifecycle dependency).
- [1 mark] for defining Aggregation (weak relationship, independent lifecycles).
- [1 mark] for identifying that the relationship here is Aggregation.
- [1 mark] for justifying that Box objects can exist independently of any specific StorageShelf (e.g., they can be moved to another shelf, truck, or floor).

Part (b): [5 Marks]
- [1 mark] for correct method header public boolean addBox(Box newBox).
- [1 mark] for implementing a loop iterating through the length of the shelves array.
- [1 mark] for executing a null check condition on current elements: shelves[i] == null.
- [1 mark] for correctly assigning shelves[i] = newBox inside the block and returning true immediately.
- [1 mark] for returning false after the loop completes if no null slot was found.

Part (c): [6 Marks]
- [1 mark] for initializing state variables to track the maximum weight (e.g., -1.0) and the index of the heaviest Box (e.g., -1).
- [1 mark] for a correct loop through the entire shelves array.
- [1 mark] for verifying that the slot is not null prior to checking its properties.
- [1 mark] for correctly using getWeight() to compare and update the maximum weight and tracked index.
- [1 mark] for checking if a valid index was found, saving the reference, and clearing the slot (setting it to null).
- [1 mark] for returning the correct Box reference, or returning null if no Box was found on the shelf.