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the cities have been changing quickly these days. about ten years ago, city is a place where you can see trees and flowers everywhere. there were buses, bikes and cars running on roads, but you rarely saw the traffic jams. the sky was blue and the air was clear and fresh. in the countryside the environment was even better! however, with the fast developing industries and increasing population, more and more people have moved into cities which has brought too much wastes into the land, air and water. now, the clear blue sky is rarely seen, and the air is totally ruined. we call the misty and grey air "smog", which means it is composed of smoke and fog. people aree still rushing into cities, but we could never go back to former days.