Life today has brought about new problem. One of the biggest problems of every nation is pollution. In America, there are laws 66 the air and water clean. These are new laws and it has taken much time 67 a polluted river or lake. Now the fish are healthy again and people can enjoy 68 in the clean water.
When food is brought from the store to the home, it is packed in paper, glass, or plastic. Every day a newspaper es to the home. Americans use 69 all these materials in the garbage which was picked up by a large truck. The truck these materials to a garbage dump or a land-fill. The truck dumped the garbage into a large hole and then earth was pushed over the garbage until the hole was filled. Then, a new hole must be found.
As the years went by, the garbage increased as the population 70 . Scientists suggested using some garbage again. They advised businessmen 71 money by recycling or using paper, glass, and plastic again.
Now, a recycling truck es to each home once a week 72 up materials. American families are told 73 their newspaper in boxes and clean all glass, and plastic. The truck takes them to factories for recycling.
Recycling is a way 74 the environment from pollution by 75 many materials again.to keep
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(答案→)to keep
解析：
【小题1】动词不定式作后置定语，修饰laws。故填to keep。
【小题2】it是形式主语，真正的主语是后面的动词不定式。清理被污染的河流要花很长时间。故填to clean。
【小题3】enjoy doing sth.“喜欢做某事”。现在，人们又可以在河里游泳了。故填swimming。
【小题4】used to do sth.“过去常常做某事”。故填to put。
【小题5】“随着人口的增长，垃圾也在增多。”，根据increased可知，应该用过去式。故填grew。
【小题6】advise sb. to do sth.“建议某人做某事”。科学家建议商人通过循环使用来赚钱。故填to make。
【小题7】不定式表示目的，pick up“捡起”。故填to pick。
【小题8】tell sb. to do sth.“告诉某人做某事”。故填to collect。
【小题9】a way to do sth.“一种做某事的方法”；save...from...“保护......不受......”。故填to save。
【小题10】by“通过”，是介词，后加动名词。通过垃圾的再次利用来保护环境。故填using。