A、Giving a lecture. B、Discussing physics problems. C、Waiting for a friend. D、Reading twentieth-century literature. B

M: Hi, Susan. Where were you at lunch time? I was saving a seat for you in the canteen.
W: Oh, sorry to miss you. But my thirst for knowledge was greater than my hunger for food.
M: Ha. I never had that problem. So where were you?
W: My physics class ran overtime.
M: That’s been happening quite a bit lately, hasn’t it?
W: I guess so. Actually what happens is that a bunch of us hang around for a while after class to talk with our professor and ask him questions.
M: Oh, the professor is m popular. Who is he?
W: Professor Hall. Have you heard of him?
M: Hm, he does have a good reputation in the physics department.
W: And a well deserved one. The same students who fall asleep in the discussion group in seminars fight for front row seats in his lecture.
M: Oh, no. I hope he isn’t catching.
W: You can joke. But it’s great to have a professor who is not only interesting, but is prepared to give up time for students.
M: I know. There I really agree. Maybe I should sit in on his class some time. Do you think he would care?
W: Not at all! Lots of students bring their friends and he says he feels flattered.