基础训练篇（16）

We must have water to live. __ 1________the water we use is clean and fresh, it will make us ill. The animals __ 2________ (live) in and around lakes and rivers must also have clean water. Our rivers and lakes supply most of our water. The clear, blue water found by the first American settlers has changed. Many of our water sources are now __ 3________. They are filled __ 4________ dirt and waste which cannot be cleaned out. In many places, water is not safe for us to use.

Lake Erie was __ 5________ a beautiful lake. Now it has been called “a dying lake”. Waste from nearby homes and factories has been dumped into __ 6________. Fish can no longer live in much of its polluted water. George Washington once called the Potomac “__ 7________ finest river in the world”. Now its water is no longer blue. Instead, it is soupy green. Suds(肥皂泡沫), from detergents（洗涤剂；洗衣粉） used to wash clothes, often cover the water of our rivers.

Water cannot __8___________(make) in a factory. Once water is polluted, it is __ 9________ to make it clean and safe again. We must learn to take care of the water __ 10_______ we now have.

离心式压缩机的密封氮气压力下降后，可采取的措施有（）。

A.可使用装置自备的钢瓶氮气

B.可使用有油氮气补充

C.可使用净化风代替

D.作将转速处理