Trees help keep the air clean and they make oxygen that we can breathe.
树木有助于净化空气，释放出可供我们呼吸的氧气。
它们帮助保持空气干净。
Trees help keep the air clean.
树木有助于空气保持清新。
They are encouraged to use bicycles, which are thought to help keep the air clean.
鼓励人们使用有助空气清洁的自行车。
They are encouraged to use bicycles, which are thought to help keep the air clean.
鼓励人们使用有助空气清洁的自行车。
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