Air is very important to everything that lives. But the air we breathe is g{#blank#}1{#/blank#} dirty. People all over the world are looking for w{#blank#}2{#/blank#} to make it clean again.

    Have you e{#blank#}3{#/blank#} watched the smoke from chimneys(烟囱) rising high in the air? Have you s{#blank#}4{#/blank#} the smoke from cars and trucks on roads or the smoke from planes? The smoke is not g{#blank#}5{#/blank#} to breathe. If we want to keep healthy and strong, we need to breathe c{#blank#}6{#/blank#} air.

    W{#blank#}7{#/blank#} more and more factories everywhere and more and more cars, trucks and planes, it is getting harder and harder to do with the air p{#blank#}8{#/blank#}.

    Scientists have a{#blank#}9{#/blank#} taken actions to stop or slow down some of the things that make the air dirty. They will not stop working u{#blank#}10{#/blank#} we have clean air again.