题干

下列四个实验中,能说明“发声的物体在振动”的实验是(   )

A:放在钟罩内的闹钟正在响铃,把钟罩内空气抽出一些后,铃声明显减小

B:使正在发声的音叉接触水面,水面溅起水花

C:吹笛子时,手指按住不同的孔便会发出不同的声音

D:在吊着的大钟上固定一枝细小的笔,把钟敲响后,让纸在笔尖上迅速滑过,可以在纸上画出一条来回弯曲的细线

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B,D

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    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 dumpsite. Residents or trash haulers(垃圾托运者) would transport household rubbish, rotted wood, and old possessions to the site. Periodically(定期的) 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, mills, and other industrial sites also had waste to be disposed of. Those located on rivers often just dumped 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, which are now called landfills, are most needed in heavily populated areas. Such areas rarely have empty land suitable for this purpose. Property is either too expensive or too close to residential(住宅区的)neighborhoods. Long-distance trash hauling has been a common practice, but once farm areas are refusing to accept rubbish from elsewhere, cheap land within trucking distance of major city areas is almost nonexistent.

    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.