听力原文：W: Hi, Tom. M: Judy, I haven't seen you in weeks. Where have you been? W: In Florida. M: What, vacationing while the rest of us have been studying on campus in the February cold. W: Not exactly, I spent most of my time underwater. M: I don't understand. W: I was on a special field trip. I went with my marine biology class. M: So you went diving. What were you looking for, sunken treasure? W: You might say that. The sea is full of treasures, all kinds of strange fascinating organisms. Our class concentrated on studying plankton. M: I thought plankton was too small to be seen. W: That's a common misconception. The term Plankton covers a wide variety of freely floating plants and animals from microscopic one-celled organisms to large ones such as the common jellyfish. M: Jellyfish may be large enough to be seen, but they are transparent, aren't they? W: Yes, most plankton have transparent tissues as a protective cover-up. It makes them practically invisible to predators. M: But not invisible to your biology class, I hype. W: By concentrating, I was able to see the outlines of lots of different plankton, plants and animals. In fact, our professor even took photographs of small oceanic snails. M: That sounds like an interesting trip. But I think if I'd been in Florida in February, I'd much rather spend my time just swimming and lying in the sun. How did Judy spend most of her time in Florida? (20)