题干

下列关于质点的说法中,正确的是(  )

A:质点是一个理想化的模型,实际并不存在,所以没有研究价值

B:因为质点没有大小,所以与几何中的点没有区别

C:凡是轻小的物体,都可看作质点

D:如果物体的形状和大小在所研究的问题中属于无关或次要因素,就可以把物体作为质点

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答案(点此获取答案解析)

D

同类题2

根据短文理解,选择正确答案。

    Disposing (处理) of waste has been a problem since humans started producing it. As more and more people choose to live close together in cities,the waste disposal problem becomes increasingly difficult.

    During the eighteenth century,it was usual for several neighboring towns to get together to select a faraway spot as a dump site. People would transport household rubbish,rotted wood and old possessions to the site. Regularly some of the trash was burned and the rest was buried. The unpleasant sights and smells caused no problem because nobody lived close by. Factories and some other industrial sites also had waste to be disposed of. Those located on rivers often just poured the unwanted remains into the water. Others built huge burners with chimneys to deal with the problem.

    Several facts make these choices unacceptable to modern society. The first problem is space. Dumps(垃圾场), are most needed in heavily populated areas. Such areas rarely have empty land suitable. Farm areas are refusing to accept rubbish from elsewhere,cheap land in major city areas to dispose wastes is nonexistent(不存在的). Awareness of pollution dangers has resulted in more strict rules of waste disposal. Pollution of rivers,ground water,land and air is a price people can no longer pay to get rid of waste. The amount of waste,however, continues to grow.

    Recycling efforts have become commonplace,and many towns require their people to take part. Even the most efficient recycling programs,however,can hope to deal with only about 50 percent of a city's reusable waste,so more people should be involved in recycling programs.