A、Biology. B、Photography. C、Swimming. D、Painting. A本题是细节题。从I went with my marine biology class．可以得知是biology。

W: Hi, Tom.
M: 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 jelly fish.
M: Jelly-fish 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 photo of some 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.