听力原文：M: I'm really shocked at the air quality of the inland valley. Look at it. W: Yes, over the last few decades there has been an enormous emphasis on development with hardly any concern for the environmental consequences. M: I can certainly see the problems it has caused. I remember watching the stars at night as a child; it was so bright and beautiful. What do you think caused all of this? W: It wasn’ t one single source like a power plant or some large industry. I' m sure there were a number of contributing factors. I bet cars were one of the major causes of damage, though. M: I remember now. Haven' t I heard something about high levels of nitrous oxides emitted from cars? W: Precisely. There are too many nitrous oxides coming out of cars. Anytime something is burned in the atmosphere at high temperature, it causes the nitrogen in the air to combine with oxygen--forming nitrous oxide radicals. Radicals are combinations of dements that behave as if they were a single element. Nitrous oxides also tend to be rather heavy. M: Well, what's wrong with that? W: When heavy gasses are released, riley don' t float away and mix with the atmosphere, they hover above the area in which they are produced causing the air to stagnate. Eventually other pollutants such as dust and soot are also mixed into the air, and a big brown cloud hangs over the city. M: Oh, I see. Then as a result of all the trapped pm-tides, the air becomes ugly, and it is difficult to see very far. W: Not only that, the nitrous oxides themselves are dangerous. Nitrous oxide radicals are highly active. They cause damage to concrete, and they are believed to create many health problems. Some people even say that certain forms of cancer may be developed from living in areas with high levels of nitrous oxides. What is the main topic of the conversation?