W: Well, that’s an assumption many people make. But the fact is bones are made of dynamic living tissue that requires continuous maintenance and repair.

　　M: Right. That’s one of the things I found so fascinating about the article the way the bones repair themselves.

　　W: Ok. So can you tell us how the bones repair themselves?

　　M: Sure. See, there are two groups of different types of specialized cells in the bone that work together to do it. The first group goes to an area of the bone that needs repair. This group of cells produce the chemical that actually breaks down the bone tissue, and leaves a hole in it. After that the second group of specialized cells comes and produces the new tissue that fills in the hole that was made by the first group.

　　W: Very good. This is a very complex process. In fact, the scientists who study human bones don’t completely understand it yet. They are still trying to find out how it all actually works. Specifically, because sometimes after the first group of cells leaves a hole in the bone tissue, for some reason, the second group doesn’t completely fill in the hole. And this can cause real problems. It can actually lead to a disease in which the bone becomes weak and is easily broken.

　　M: Ok, I get it. So if the scientists can figure out what makes the specialized cells work, maybe they can find a way to make sure the second group of cells completely fills the hole in the bone tissue every time. That’ll prevent the disease from ever occurring.

　　19. What is the discussion mainly about?

　　20. What is the function of the first group of specialized cells discussed in the talk?

　　21. What does the professor say about scientists who study the specialized cells in human bones?

　　22.According to the student, what is one important purpose of studying specialized cells in human bones?

　　Conversation 2

　　答案：CBD

　　原文：

　　M: Hi Diana, mind if I sit down?

　　W: Not at all, Jerry. How have you been?

　　M: Good. But I’m surprised to see you on the city bus. Your car in the shop?

　　W: No. I’ve just been thinking a lot about the environment lately. So I decided the air will be a lot cleaner if we all use public transportation when we could.

　　M: I’m sure you are right. The diesel bus isn’t exactly pollution free.

　　W: True. They’ll be running a lot cleaner soon. We were just talking about that in my environmental engineering class.

　　M: What could the city do? Install pollution filters in all their buses?

　　W: They could, but those filters make the engines work harder and really cut down on the fuel efficiency. Instead they found a way to make their engines more efficient.

　　M: How?

　　W: Well, there is a material that’s a really good insulator. And a thin coat of it gets sprayed on the certain part of the engine.

　　M: An insulator?

　　W: Yeah. What it does is reflect back the heat of burning fuel. So the fuel will burn much hotter and burn up more completely.