Question

Which verbs agrees in number with its subject? The cost of eggs ........... about three dollars a dozen.

Answer 1

Since "the cost of eggs" is singular, a. is agrees with it and is the correct answer.

Good Luck 

Answer 2

The subject of the sentence, 'The cost (of eggs)" is singular, so we need a verb in singular too. the only such verb is a. is, so that's the correct answer.