Given the task definition and input, reply with output. You are given a sentence, a question and two answer options ('A' and 'B'). Your task is to find the correct answer (return the string of the correct option, not 'A' or 'B') for the given question.

Sentence: A dirty tooth isn't as smooth as a clean tooth. Question: Which surface will a toothbrush move over at a greater rate of speed? (A) dirty tooth (B) clean tooth
clean tooth