Which sentence combines these two sentences and includes a relative clause?

The twins pretended to be each other. The switch was a huge mistake.

Choices

The twins pretended to be each other, which was a huge mistake.

The twins pretended to be each other, a huge mistake.

The twins pretended to be each other because they made a huge mistake.

The twins pretended to be each other, and it was a huge mistake.

I looked up relative clause, but still not sure I understand what a relative clause is.

I am thinking the first sentence, but the last one might be the better choice. Just not sure from what I looked up.

the first one is correct. It contains a relative clause

#3 also has a subordinate clause, but not a relative clause. Relative clauses are clauses starting with the relative pronouns who/whom, that, which, whose, where, when.

The last one is just a compound sentence, with two independent clauses separated by "and"

Yes, the first sentence has a relative clause.

This is my favorite grammar site.

http://www.chompchomp.com/terms/relativeclause.htm

Thank you both and thanks for the link

A relative clause is a type of subordinate clause that provides additional information about a noun in the main clause. It is typically introduced by a relative pronoun (e.g., who, which, that, etc.) or a relative adverb (e.g., where, when, why, etc.).

In the given sentences, the correct sentence that combines them and includes a relative clause is:

The twins pretended to be each other, which was a huge mistake.

In this sentence, the relative pronoun "which" introduces the relative clause "which was a huge mistake," providing additional information about the action of the twins pretending to be each other. The relative clause is set off by commas, indicating that it is not essential to the main meaning of the sentence.

While the other choices also include the two sentences, they do not properly utilize a relative clause. The second choice lacks a relative clause altogether. The third choice uses a dependent clause starting with "because" instead of a relative clause. The fourth choice uses a coordinating conjunction ("and") without a relative clause.

Understanding the concept of a relative clause will help you identify when it is being used in sentence structure.