听力原文：M: I'm really sorry, Professor Irwin. I was sick yester-day. W: Look, I'm not upset that you couldn't keep the appointment, but it is common courtesy to call. You know that. M: Yes, I do. W: During registration I have to see all my students, and sometimes they have to wait several days to get in. When someone doesn't show and doesn't call, that deprives someone else of an appointment time. M: You're right. I apologize. I didn't feel well, and I guess I just wasn't thinking straight at the time. W: Okay. Apology accepted. Now, I suppose you need to set up another appointment. M: Yes, I do. Can you see me now if I wait? W: No. I can see yon at three o'clock this afternoon, or during my office hour on Tuesday or Thursday. M: Great. Your office hour is best, That's two o'clock, right? W: That's right. Which day do you prefer? M: Tuesday. W: Okay. Be there this time. M: I will be. Thanks a lot. What is the main subject of the conversation?