A、Biology. B、Photography. C、Swimming. D、Painting. A由对话中女士提到的I went with my marine biology class可知，答案为A)。

W: Hi, Tom.
M: Hi, Kate. I haven’t seen you for weeks. Where have you been?
W: In Florida.
M: What? Vacationing while the rest of us have been studying on the 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 scuba diving. What were you looking for, sunken treasure?
W: You might say so. The sea is full of treasures, all kinds of strange, fascinating organisms. Our class concentrated on studying plankton.
M: I thought they were too small to be seen.
W: That’s a common misconception. The plankton covers a wide variety of freely floating plants and animals, from 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 floating plants and animals have transparent tissues. It makes them practically invisible to their enemies.
M: But not invisible to your biology class, I hope.
W: By concentrating, I was able to see the outlines of lots of different plants and animals. In fact, our professor even took photos of some of the small oceanic snails.
M: How would the snails show up in the photo if they are transparent?
W: We painted them with a harmless green dye. Since particles of the dye stuck to the tissues, the snails appeared in a green outline in the photos.
M: That sounds like an interesting trip.
W: It really was.
M: But I think if I had been there, I’d much rather have spent my time just swimming and lying in the sun.
Questions 23 to 25 are based on the conversation you have just heard.
23. How did Kate spend most of her time in Florida?
24. What kind of class went on the field trip?
25. According to Kate, what did her professor photograph?