（    ）45．A．changed         B．kept             C．taught

A

Air is very important to everything that lives．But the air we breathe（呼吸）is getting dirty．People all over the world are looking for ways to make it clean again.

Have you ever watched the smoke from chimneys（烟囱）rising high and spreading（散布）out in the air? Or seen the smoke that puffs（喷出）from cars and trucks on the road? Or watched the smoke trailing（从后面喷出）from a jet plane?

    Sometimes smoke looks pretty in the air．But it is not good to breathe．If we are going to keep well and strong，we need to breathe clean air．

    With more and more factories everywhere and more and more cars，trucks and planes，it is getting harder and harder to keep our air clean．

    Scientists have already found ways to stop or slow down some of things that ulake the air dirty．They will not stop working till we have clean air again．