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### Theoretical Explanation

A **fixed exchange rate** is a regime where a country's currency value is pegged to another currency or a basket of currencies by the central bank. When there is downward pressure on the currency (e.g., due to a trade deficit, speculative capital outflows, or high domestic inflation relative to trading partners), the market exchange rate threatens to fall below the pegged level. This creates a surplus of the currency at the fixed rate.

To maintain the peg, the central bank has two primary tools:

1. **Buying Domestic Currency using Foreign Exchange Reserves:**
The central bank sells its reserves of foreign currencies (such as US Dollars or Euros) and buys its own currency in the foreign exchange market. This increases the market demand for the domestic currency, shifting the demand curve to the right, which counteracts the downward pressure and keeps the exchange rate at the pegged value.

2. **Increasing Domestic Interest Rates:**
The central bank can implement tight monetary policy by raising domestic interest rates. Higher interest rates attract financial investment ("hot money") from abroad as foreign investors seek higher returns. To invest, they must buy the domestic currency, which increases demand for the domestic currency and reduces its supply (as domestic residents keep their money at home), stabilizing the exchange rate.

### Diagrammatic Representation

In a foreign exchange diagram:
- The vertical axis represents the exchange rate in terms of foreign currency per unit of domestic currency (
\(P\) of domestic currency in foreign currency).
- The horizontal axis represents the quantity of domestic currency.
- Let the initial equilibrium be at the pegged rate \(ER_{peg}\), where market demand \(D_1\) meets supply \(S_1\).
- If downward pressure occurs (e.g., due to capital flight, shifting demand from \(D_1\) to \(D_2\)), a surplus of the currency is created at \(ER_{peg}\).
- The central bank's intervention to buy domestic currency shifts demand back from \(D_2\) to \(D_1\) (or a new demand curve that intersects \(S_1\) at \(ER_{peg}\)), restoring equilibrium at the pegged exchange rate.

### Real-World Example

- **Hong Kong Monetary Authority (HKMA):** The Hong Kong Dollar (HKD) is linked to the US Dollar (USD) at a fixed range of 7.75 to 7.85 HKD per USD. When there is downward pressure on the HKD (pushing it toward the weak end of 7.85), the HKMA intervenes by buying HKD and selling USD from its massive foreign exchange reserves, while simultaneously allowing local interbank interest rates (HIBOR) to rise, thereby defending the peg successfully.

**Markband Descriptor (Out of 10 Marks)**

- **Level 4 (9–10 marks):** The response shows an in-depth understanding of the demands of the question. A relevant economic diagram is drawn and fully explained. Relevant economic terms (fixed exchange rate, central bank intervention, foreign exchange reserves, interest rates) are defined and used accurately. A highly relevant real-world example (e.g., HKMA defending the HKD) is integrated and explained clearly to support the theoretical explanation.

- **Level 3 (7–8 marks):** The response shows a good understanding of how a central bank defends a fixed exchange rate. An appropriate diagram is drawn, though there may be minor omissions in labeling or explanation. Key concepts are explained, including buying currency and changing interest rates, but the explanation may lack some depth. A relevant real-world example is included but not fully integrated.

- **Level 2 (4–6 marks):** The response shows some understanding of the mechanisms used. The diagram is partially complete or contains errors. The distinction between buying reserves and raising interest rates is vague. Real-world examples are either missing, inaccurate, or merely mentioned without development.

- **Level 1 (1–3 marks):** The response is very limited. Important concepts are missing, incorrect, or heavily confused. No relevant diagram or real-world example is provided.

Marking Scheme (Max 15 marks):

• 13–15 marks: The candidate demonstrates excellent understanding of monetary unions. Relevant diagrams (AD/AS) are fully integrated, correctly drawn, and explained. There is a well-structured, balanced discussion of both the benefits (transaction costs, transparency, trade) and costs (loss of monetary/exchange rate autonomy). Real-world examples (such as the Eurozone crisis) are used effectively. Evaluation is critical, synthesized, and draws upon economic theories like the Optimum Currency Area (OCA).

• 10–12 marks: The candidate provides a good explanation of the benefits and costs of joining a monetary union. Relevant diagrams are included but may not be fully integrated into the text. Examples are used but may lack detail. Discussion is balanced, but the final evaluation is less developed or lacks synthesis.

• 7–9 marks: The candidate explains some benefits and costs but the discussion is unbalanced or lacks depth. Diagrams may be missing or contain errors. Examples are weak or absent. Evaluation is superficial or merely a summary.

• 4–6 marks: The candidate shows limited understanding of monetary unions. Terminology is weak, and there is little to no evaluation.

• 1–3 marks: The response is largely irrelevant, with major misconceptions about economic integration.

Marking Scheme (Max 15 marks):

• 13–15 marks: The candidate demonstrates excellent understanding of monetary unions. Relevant diagrams (AD/AS) are fully integrated, correctly drawn, and explained. There is a well-structured, balanced discussion of both the benefits (transaction costs, transparency, trade) and costs (loss of monetary/exchange rate autonomy). Real-world examples (such as the Eurozone crisis) are used effectively. Evaluation is critical, synthesized, and draws upon economic theories like the Optimum Currency Area (OCA).

• 10–12 marks: The candidate provides a good explanation of the benefits and costs of joining a monetary union. Relevant diagrams are included but may not be fully integrated into the text. Examples are used but may lack detail. Discussion is balanced, but the final evaluation is less developed or lacks synthesis.

• 7–9 marks: The candidate explains some benefits and costs but the discussion is unbalanced or lacks depth. Diagrams may be missing or contain errors. Examples are weak or absent. Evaluation is superficial or merely a summary.

• 4–6 marks: The candidate shows limited understanding of monetary unions. Terminology is weak, and there is little to no evaluation.

• 1–3 marks: The response is largely irrelevant, with major misconceptions about economic integration.