18. A restrictive clause is one that

A. should be set off by commas.

B. usually occurs at the beginning of a sentence.

C. limits the meaning of the word it describes.

D. functions as an adverb.

The answer is C. limits the meaning of the word it describes.

It should not be set off by commas, it can occur at the beginning or the end of a sentence, and doesn't function as an adverb.

Example: The man who lives next door is old.

This restrictive clause limits "the man" to say it's specifically the one next door.