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 studying on the campus in February cold？
W: Not exactly. I spent most of my time under water.
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's full of treasures. All kinds of strange fascinating organisms. Our class concentrated on studying plankton.
M: I found plankton were too small to be seen.
W: That's a common misconception. The term plankton covers a wild variety of freely flowing plants and animals, from microscopic one cell organisms to larger 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 planktons have transparent tissues as protected camouflage, it makes them practically invisible to predators.
M: But not invisible to your biology class, I hope.
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 gastropods, which are small oceanic snails.
M: How would the snails show up in the photographs of their transparence？
W: We scoured it with harmless green dye since particles of the dyes stuck to their tissues, the snails appeared in green outline in the photographs.
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.
(20)

A. She had her vacation there.
B. She took a diving course there.
C. She was there on a field trip.
D. She visited a marine exhibition.