A、Wife and husband. B、Teacher and student. C、Clerk and guest. D、Employer and employee. C根据上一题的信息可推知他俩是客户与代理人之间的关系，故选C。

M: Good morning. Will you book a plane ticket to London for me?
W: Yes, sir. What time would you like to leave?
M: Next Tuesday, if possible.
W: That’ll be the 21st of December.
M: Yes, that’s right.
W: There are several flights available.
M: Would you check what flights are available on the 21st of December?
W: All right. Let me check it over.
M: Morning flights will be perfect.
W: Yes, sir. There is a flight available on that day, at 10: 00 a. m.
M: Fine. I’ll take that flight, then.
W: 10: 00 a. m. I’ll fix your ticket.
M: When do I check in?
W: You must be there by 9: 00 a. m.
M: Thank you.
18: What does the conversation talk about?
19: What’s the relationship between the two speakers?