A、A crowded city. B、A steel factory. C、A busy airport. D、An old car park. A

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.