听力原文：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 up 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 nest 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? 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. M: Well, you've still got time. It sounds like that the bees are picky about who comes to their family reunion. What is the relationship between the speakers?