Given the task definition and input, reply with output. In this task, you will be shown a conversation and a question. You need to answer the question and choose the correct option based on the conversation. "W" and "M" in the conversations stand for "woman" and "man".

W: John, do you want to go swimming with me today? M: Sure, but I can't leave now. I have an appointment with my professor at 3o'clock., Question: Why can't John go swimming now? (A) He must meet his teacher. (B) He must attend a class. (C) He must go out with his girlfriend.
(A) He must meet his teacher.