听力原文：M: Math department, Professor Webster speaking.
W: Hello, Prof. Webster. This is Janet Hill calling. I'm a neighbor of your teaching assistant, Don Williams. Don asked me to call you because he has lost his voice and can't talk to you himself.
M: Oh, I'm sorry. Is there anything I can do for him？
W: Well, he has a class this afternoon at two-thirty to four and he won't be able to teach it. But he doesn't want to cancel it either.
M: Does he want to find somebody else to teach the class？
W: No, not exactly. He wants to get someone to go in for him, just to pass back the mid-term papers. He's already marked them and they are on the desk in his office. The whole thing wouldn't take more than ten minutes.
M: His classes are at two-thirty, right？ Well I'm free at that time, so I could do it for him. What room is his class in？
W: Cater Hall, Room 214. Will you need his office key？ He's given it to me and I could bring it to you.
M: Actually, that won't be necessary. We have a master key in the math department. So I can get into his office.
W: Thank you very much, Prof. Webster. Don doesn't have another class to teach until Thursday, and hopefully, he will be able to talk to you by then. He'll call you as soon as he can. Oh, yea, I almost forgot.
M: Yes？
W: Could you put the next assignment on the board, too？ It's all the problems on page forty-five, and they are due at the next class.
M: No trouble at all. Thanks for passing on the news about Don, and please tell him not to worry about anything.
(23)

A. He is unable to talk.
B. He doesn't like his classroom.
C. He can't find his office key.
D. He has misplaced some exams.