初中作業(yè).
Some cities in the world were polluted badly,the most famous ones are Milan(Italy),Pittsburgh(USA) and Mexico City(Mexico).For Milan,because there are too many cars,so the air were dirty and the suspended particles makes smoke in the whole city.Pittsburgh is one of the two worst-polluted cities in America,the cause of its pollution is heavy metals,the smoke,and polluted air come from Los Angeles.And because of its geographical environment,the air in Mexico City were blocked,and there are over a million cars in that city,so the air is very dirty.