语音讲解：
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　　Water is the "life food" of our earth. It is in every living thing. It is in the air. It runs through mountains and valleys. It forms lakes and seas. Nature has a great water system. Rainwater finds its way to rivers and lakes. River water runs into the sea. At the mouths of the rivers, fresh water joins the salt water of the sea.Here at the mouth of a river there is much important plant and animal life. But pollution destroys this life. We have to clean our rivers. Man has to work with nature, not against it.