A、They’re cousins. B、They’re lab partners. C、They’re classmates. D、They’re roommates. C浏览选项可知，本题考查对话双方之间的关系。对话中女士和男士互相打招呼，接着二人谈到

M: Hi, Sue. Where have you been?
W: Oh, hi, Dan. I was just at the library. I have to hand in my biology paper tomorrow.
M: Tomorrow? Oh, no! I thought it wasn’t due till Monday.
W: Oh, don’t worry. It is due Monday. But I’m going away for the weekend and won’t be back till Monday night.
M: Oh, where are you going?
W: California. We’re having a family reunion. It’s my grandmother’s ninetieth birthday. So all the cousins and aunts and uncles are going. She planned the whole thing herself.
M: Wow, that’s great. How many people will be there?
W: Around sixty. My family is big and spread out but we’re pretty close. So have you started working on your biology paper?
M: Yeah, I’m doing it on bees and how they’re able to recognize whether another bee is related to them.
W: How can they tell?
M: They use their sense of smell. The sweat bees guard their nests this way. If another bee approaches the nest, the guard determines if the new bee is familiar. If it is, it’s allowed to enter.
W: Interesting. Can other insects do this?
M: Well, the paper wasps can. Each wasp nest has a special combination of plant fibers and so the wasps that live there have a unique smell. Those two are the only kinds I’ve read about so far.
W: Well, you’ve still got time. It sounds like the bees are picky about who comes to their family reunion.
Questions 19 to 22 are based on the conversation you have just heard.
19. What is the relationship between the speakers?
20. Why was the man worried at first?
21. According to the man, what do some bees use their sense of smell for?
22. What will the man probably do over the weekend?