What was Tom doing in February?

Judy: Hi, Tom!
Tom: Judy. Haven’t seen you in weeks. Where have you been?
Judy: In Florida.
Tom: (9[C]) What, going On holiday! While the rest of us are studying on the campus in February cold?
Judy: Not exactly. I spent most of my time under water.
Tom: I don’t understand.
Judy: (10[A]) I was on a special field trip. I went with my marine biology class.
Tom: So you went diving. What were you looking for, sunken treasure?
Judy: You might say so. The sea’s full of treasures. All kinds of strange fascinating organisms. Our class concentrated on studying plankton.
Tom: I thought planktons were too small to be seen.
Judy: That’s common misconception. The term plankton covers a wild variety of freely flowing plants and animals, from one cell organisms to larger ones, such as the common jellyfish.
Tom: Jellyfish may be large enough to be seen. But can you see the very tiny planktons?
Judy: By concentrating. I was able to see the outlines of lots of different plankton plants and animals.
Tom: That sounds like an interesting trip. But I think if I’ve been in Florida in February, I’d much rather spend my time just swimming and lying in the sun.