听力原文：W: Hi, Mike. M: Hi. I'm surprised to see you on the city bus. Why not drive your car? W: (23)I've been thinking about the environment lately. If we all use public transportation when we could, the air will be much cleaner. M: Right. But the bus isn't exactly pollution free. W: True. But they'll be running a lot cleaner soon, We were just talking about that in my environmental engineering class. M: What's the city going to do? Install pollution filters of some sort on their buses? W: They could, but those filters make the engines work harder and really cut down on fuel efficiency. Instead they found a way to make their engines more efficient. M: How? W: Well, (24)there is a material called the coniine oxide. It's a really good insulator. And a thick coat of it gets sprayed on the certain part of the engine. M: An insulator? W: Yes. (25)It reflects 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, right? W: Yeah, and the bus will need less fuel. So with the savings 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 this stuff to spray their car engines. W: Well, not really that easy. To melt the materials before you can spray a coat of it on the engine parts, you first have to heat it over 10,000 degrees. It's not something we are able to do ourselves. (20)