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问题 选择题

日光灯镇流器的作用有

A．启动器动、静片接触时，镇流器产生瞬时高压

B．正常工作时，镇流器降压限流保证日光灯正常工作

C．正常工作时，使日光灯管的电压稳定在220V

D．正常工作时，不准电流通过日光灯管

答案

答案：B

日光灯镇流器启动时产生高压，正常工作时起降压限流保证日光灯正常工作，B正确。

单项选择题

在城市工程管线综合术语中，（）是指地面到管道顶的距离。

A.管线垂直净距

B.管线覆土深度

C.管线埋设深度

D.管线水平净距
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完形填空

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.
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