A、They are microscopic. B、They move very quickly. C、They are transparent. D、They are rarely found near the surface. C

Judy: Hi, Tom!
Tom: Judy. I haven’t seen you in weeks. Where have you been?
Judy: In Florida.
Tom: What vacationing! While the rest of us is studying on the campus in February cold?
Judy: Not exactly. I spent most of my time under water.
Tom: I don’t understand.
Judy: I was on a special field trip. I went with my marine biology class.
Tom: So you went scuba 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 microscopic one cell organisms to larger ones, such as the common jellyfish.
Tom: Jellyfish maybe large enough to be seen. But they are transparent, aren’t they?
Judy: Yes, most planktons have transparent tissues as protected camouflage, it makes them practical in visible to predators.
Tom: But not invisible to your biology class, I hope.
Judy: 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 Greeber Quadata which are small oceanic snails.
Tom: How would the snails show up in the photographs if they are transparent?
Judy: We scoured it, Greeber Quadata, with harmless green dye. Since particles of the dyes stuck to their tissues, the snails appeared in green outline in the photographs.
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.