Prepositions before relative pronouns：练习题

When using a preposition before a relative pronoun in formal writing, which of the following sentences is grammatically correct?

Choose the correct preposition and relative pronoun to complete the sentence:
The project ______ they are working is extremely complicated.

Identify the sentence that uses a formal structure with a preposition before the relative pronoun.

Combine these two sentences using the most formal structure:
This is the theory. Scientists have been debating it for years.

In formal written English, when a preposition is used before a relative pronoun to introduce a clause, which of the following sentences is grammatically correct and most appropriate for a formal context?

Rewrite the following sentence to make it more formal by moving the preposition before the relative pronoun:

This is the neighbor whom I received the letter from.

Formal English often requires a preposition before a relative pronoun. Combine the following two sentences into one formal sentence using the relative pronoun which:

The experiment was successful. Many students participated in it.<\/i><\/p>

Transform the following two sentences into one single formal sentence using a relative pronoun and a preposition. Then, state whether your new sentence contains a defining or non-defining clause.

We are waiting for the bus. The bus is late.