But it is-this sentence has a complete main clause caffeine is consumed and an incomplete main clause also found in the incomplete clause needs a subject and a helping verb to be complete, and the sentence needs a coordinate connector to join the two clauses. answer c is the best answer because it contains the subject it, the helping verb is, and the coordinate connector but. answer a is missing a subject and a helping verb, answer b is missing a helping verb, and answer d is missing a coordinate connector, so these answers are incorrect.