阅读下面短文，从短文后各题所给的四个选项（A、B、C和D）中选出最佳选项。
Water is the “life blood” of our earth. It is in every living thing. It is in the air. It runs through mountains and valleys. It forms lakes and oceans. Water is everywhere.
Nature has a great water system. Rain water finds its way to streams and the oceans.
Here at the mouth of a river there is much important plant and animal life. Pollution destroys this life, so we have to clean out streams and rivers. Man has to work with nature--not against it.
