A、By expelling its organs. B、By moving quickly. C、By organizing itself. D、By hiding itself. A题目问的是海参是如何保护自己的。由定位句可知，海参通过喷出自己的内脏器

W: Wasn’t that a fascinating class? (16) Of all the animals in the sea, the sea cucumber, with its bizarre self-defense mechanism, has always interested me.
M: Dr. Lambert sure knows what he is talking about, but I still don’t understand why he said that (17) the sea cucumber defends itself by spitting out its internal organs. He carefully explained its distinct method of escaping capture, (18) but I am not sure that it made sense.
W: Wasn’t that a fascinating part of the lecture! When the sea cucumber feels threatened, it strongly contracts its muscles and discharges its internal organs towards the attacker. That usually confuses the attacker enough to leave the sea cucumber alone.
M: That’ s certainly a unique way to defend yourself from danger. But isn’ t it just as bad as being attacked if you have to spit up all your internal organs?
W: Perhaps if you were a person, but the sea cucumber can grow its organs back in a few days.
And really when we talk about the organs of a sea cucumber, we are only talking about a few digestive organs. If you remember from the lecture, Dr. Lambert pointed out that (19) the sea cucumber is a very primitive animal. It doesn’t even have a spine. Animals without spines are difficult to think in human terms because their requirements for life are very minimal. For example, (20) many worms can be cut in half and they just wiggle away and grow into new worms. Obviously people don’t have those same abilities.
M: Okay, so a cucumber doesn’t really just give up its most important parts the way a human would.
W: No, not at all. A sea cucumber’s digestive organs are almost like people’ s hair. If you cut it off, it doesn’t hurt the animal.
M: It’s still a fascinating idea though. The next time somebody gives me troubles, I’ll remember what I would do if I were a sea cucumber.
16．What is the main topic of this conversation?
17．How does a sea cucumber protect itself?
18．Why was the man confused?
19．How can we tell that the sea cucumber is a primitive animal?
20．Which of the following is NOT TRUE of sea cucumbers?