听力原文：W: Air pollution is one of the most serious problems in the world. Polluted air can make us sick or even kill us. Why isn't more done to stop it? M: Everyone wants to stop it, but it is a difficult problem to solve. Most pollution is caused by things people need. W: What do you mean? M: Well, automobiles and airplanes cause pollution but they also provide transportation. W: Right. And factories cause air pollution but they provide people with jobs and products. M: Cities are the worst offenders. In crowded cities, factories and cars can add tons of pollution to the air every day. W: Yes. When I drove to town yesterday, the clear air changed. It seemed like a dark cloud hung over the city. M: Nobody likes air pollution. It smells had; it makes it difficult to breathe. W: True, k even damages the plants that provide us with {cod. No wonder we get sick. It's dangerous to everyone' s health. M: I think everyone knows it is a serious problem. When air pollution can even damage strong materials like steel and concrete; think what it does to poor humans. W: It is terrible, and yet we do need some things that cause it. We'll have to try to figure out a solution. (20)