听力原文：M: Good morning, Professor Perkins, I hope I am not disturbing you.W: Not at all.

听力原文：M: Good morning, Professor Perkins, I hope I am not disturbing you.

W: Not at all. Tom, come right in. I'm always at ,my office in the morning.

M: I thought I'd get an early start on my research paper and would like to discuss my topic with you if you have a moment.

W: Of course. I recommend all my students discuss their topics with me before they begin their research. What do you want to work on？

M: I was especially interested in your lecture on dinosaurs and the apparent mystery surrounding their extinction. I'd like to explore that question, but I'm not too sure how to go about it.

W: Well, according to the most widely held theory, the dinosaurs died out because of a sudden cooling of the earth's temperature. Your textbook summarizes the conclusions of several paleontologists on this point.

M: Didn't you also mention a second theory in your lecture that the dinosaurs may simply have been replaced by mammals gradually and might not have died out as a direct response to the colder weather？ W: Yes, Van Wehlen and Sloane are proponents of this theory, and I've out some of their articles on reserve in the library. M: Van Wehlen and Sloane. I'd better write that down. W: Let me know how your work progresses, Tom. If you should run into any problems, be sure to stop by a-gain.

What are the speakers？

A．Student-Student.

B．Advisor-Student.

C．Professor-Student.

D．Librarian-Professor.