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The Earth is polluted（被污染）much. In some cities in the world, the air is dirty. People have to wmasks on their way to school or work.

But how does this happen? The awas not so dirty before. Cars and motorbikes are one of the reasons（原因）. Nowadays, there are more and more cars and motorbikes in the s. The sfrom them makes the air very dirty. So we should susing more cars. We should take buses or the underground more often to school or work. If we go to a place nearby, walking is a good way. It can also make us hand strong. So let's do something to make the air clean.