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Air is very important to everything that lives. But the air we breathe is g   dirty. People all over the world are looking for w   to make it clean again.
Have you ever watched the smoke from chimneys(烟囱) r   high in the air? Have you s  ___ the smoke from cars and trucks on roads or the smoke from planes? The smoke is h   to everyone, especially old people and children. If we want to keep well and strong, we need to breathe f   air.
With more and m     factories everywhere and more and more cars, trucks and planes, it is getting harder and harder to do with the air p   .
Scientists have a   found ways to stop or slow down some of the things that make the air dirty. They will not stop working u  we have clean air again.