Subject-verb agreement

3a. Vitamins that are sold in a health-food store are not regulated by the Food and Drug Administration.

3b. Vitamins that are sold in a health-food store is not regulated by the Food and Drug Administration.

My answer is: 3a

Right again! :-)

what is the explanation to this answer why would you choose are over is?

Correct! The subject-verb agreement in sentence 3a is accurate. "Vitamins" (plural) is the subject of the sentence, so it should be followed by the plural verb "are." Therefore, the sentence should read: "Vitamins that are sold in a health-food store are not regulated by the Food and Drug Administration."

In contrast, sentence 3b is incorrect. It uses the singular verb "is" instead of the plural verb "are." This creates a subject-verb disagreement. Remember, when the subject is plural, the verb must also be plural. So, the correct version of 3b would be: "Vitamins that are sold in a health-food store are not regulated by the Food and Drug Administration."