Questions 19 to 22 are based on the conversation you have just heard.

19.What is the discussion mainly about?
【解析】［C］对话一开始就谈论人类骨骼的问题，女士说 But the fact is bones are made of dynamic living tissue that requires continuous maintenance and repair. 后面的对话都是针对这个话题展开的讨论，可见整个对话讨论的话题是骨骼的自我修复能力。

20.What is the function of the first group of specialized cells discussed in the talk?
【解析】［C］男士解释了第一组细胞的作用，第一组细胞生成一种化学物质分解骨组织(This group of cells produces the chemical that actually breaks down the bone tissue...)，由此可见答案是They break down bone tissue.

21.What does the professor say about scientists who study the specialized cells in human bones?
【解析】［B］女士说这种特殊细胞作用于人类骨骼的过程相当复杂，事实上，连研究人类骨骼的科学家都不能完全了解这个过程。从这一句中可以知道：In fact, the scientists who study human bones don’t completely understand it yet. 所以答案即是这些科学家也
不完全知道这些细胞如何工作。

22.According to the student, which is one important purpose of studying specialized cells in human bones?
【解析】［A］学习这些人类骨骼中的特殊细胞的重要目的是为了学习如何预防骨头疾病。从
男士的这一句话中可看出：That’ll prevent the disease from ever occurring. 这样可
以在疾病发生之前预防疾病，可见［A］符合题意。

Conversation Two
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.
M:So a lot less unburned fuel comes out to pollute the air.
W:And the bus will need less fuel. So with the saving on fuel cost, they say this will all pay for itself in just six months.
M:Sounds like people should all go out and get some this stuff to spray their car engines.
W:Well, it’s not really that easy. You see, normally, the materials are fine powder. To melt it so you can spray a coat of it on the engine parts, you first have to heat it over 10,000 degrees and then, well, you get the idea. It’s not something you or I will be able to do ourselves.