题干

将一把钢尺紧按在桌面上,一端伸出桌面适当的长度,拨动钢尺,就可听到钢尺振动发出的声音.逐渐增加钢尺伸出桌面的长度,钢尺振动发出声音的音调会逐渐变____.当钢尺伸出桌面超过一定长度时,虽然用同样的力拨动钢尺振动,却听不到声音,这是由于____

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低,声音的频率低于20Hz

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    Increasingly, over the past ten years, people-especially young people have become aware of the need to change their eating habits, because much of the food they eat, particularly processed food, is not good for the health. Consequently, there has been a growing interest in natural foods: foods which do not contain chemical additives and which have not been affected by chemical fertilizers(化肥)widely used in farming today.

    Natural foods, for example, are vegetables, fruit and grain which have been grown in soil that is rich in organic matter. In simple terms, this means that the soil has been nourished by unused vegetable matter, which provides it with essential vitamins and minerals. This in itself is a natural process compared with the use of chemicals and fertilizers, the main purpose of which is to increase the amount but not the quality of foods grown in commercial farming areas.

    Natural foods also include animals which have been allowed to feed and move freely in healthy pastures(生活状况). Compare this with what happens in the mass production of poultry(家禽): there are battery farms, for example, where thousands of chickens live crowded together in one building and are fed on food which is little than rubbish. Chickens kept in this way are not only tasteless as food; they also produce eggs which lack important vitamins.

    There are other aspects of healthy eating which are now receiving increasing attention from experts in diet. Take, for example, the question of sugar. This is actually a nonessential food! It is not that sugar is harmful in itself. But it does seem to be additive: the quantity we use has grown steadily and in Britain today each person consumes an average of 200 pounds a year! Yet all it does is provide us with energy, in the form of calories. There are no vitamins in it, no minerals and no fibre.