【正确答案】
C
【答案解析】[听力原文]19-21
M: Math department, Doctor Webster speaking.
W: Hello, Professor Webster, this is Janet Hill calling. I live two doors down from your teaching assistant, Don Williams. (19) Don asked me to call you because he has lost his voice and can't talk to you himself.
M: Oh, what a shame! Is there anything I can do for him?
W: Well, he has a class this afternoon from 2:30 to 4:00 and he won't be able to teach it. But he doesn't want to cancel it, either.
M: Does he want me to try to find somebody else to teach the class?
W: No, not exactly. What he wants to do is to get someone to go in for him. Just to pass back the mid-terra exams. And they are on the desk in his office.
M: His class is at 2: 30, you say? 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. (20) Will you need his office key to get the exams? He's given it to me and I could bring it to you.
M: Well, we have another key in the department. So I can get into his office if necessary.
W: Thank you very much, Professor Webster. Don will call you as soon as he can. Oh, yes, I almost forget.
M: Yes?
W: (21) Could you put the next assignment on the board, too? It's all the problems on page 45, and they are due at the next class.
M: No trouble at all. And please tell him not to worry about anything.
19．Why is Don unable to teach his class?
[听前猜测] 由选项推测本题的四个选项都会导致he没法上班或上课，关键确定哪一个符合题意。
[精解] 题目问Don不能上课的原因。关键信息为lost his voice，意思是说Don嗓子失声了，没法讲课，故答案为C)。