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.
Q18：What does the conversation talk about？
Q19：What’s the relationship between the two speakers？