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  3. When nature is left alone, a balance is reached among the animals and plants living in one area. But when man interferes with na

When nature is left alone, a balance is reached among the animals and plants living in one area. But when man interferes with na

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问题    When nature is left alone, a balance is reached among the animals and plants living in one area. But when man interferes with nature, the balance is upset. He grows a crop and takes it away to eat; then there are no dead leaves to fall on the ground, holding water while it sinks into the surface, or decaying and adding humus to the soil. Unless a farmer acts with knowledge and skill, he tends therefore to make the land poorer. To take the place of the organic matter in the crops that he removes, he uses some kind of fertilizer. Chemical fertilizers am very helpful, but the waste products of animals and the decaying remains of plants should also be put on the land. In some places it is a habit to bum waste material lying about, but such burning destroys the organic matter in the dead plants. Although the ashes that am left am valuable when put on the land, a better practice is to bury the waste, so that it decays and increases the humus in the soil.
   In the past when the world population was much lower than it is now, a man had little difficulty in ordinary times in growing the food that was needed. When a field had been cultivated for some years and had be come tired, the farmer could move to another place. The tired land then slowly recovered. Gradually grasses and other plants would appear on it and its productive power would be slowly restored by their decay. But nature, left alone, would take a long time to bring back the land to its former state; the length of time required would depend on local conditions, but it might well be ten years.
   It is a bad practice to grow the same crop in a field year after year. If the crop is changed, the land will suffer less because it is treated and used in different ways. Different plants affect the soil differently. Consequently a change of crop will do less harm than the growing of the same crop year after year and a periodic change to grass will benefit the soil. Much will therefore be gained ff different crops are grown one after the other, a method known as the rotation of crops.
It is implied that a modem farmer cannot simply move to another place when his field has become tired. This is because ______.
选项 A、the production power of a new field is not higher than that of an old one
B、it takes a farmer more than ten years to cultivate a new field
C、the population of the world has increased rapidly
D、there will be too many grasses in a new field to grow crops
答案C
解析 作者在文章第二段开始指出“In the past when the world population was much lower than it is now．．．When a field had been cultivated for some years and had become tired，the farmer could move to another place．”在这里作者暗示出：现在人口众多，再像过去那样通过搬迁生产食物就有很大困难了，因此选[C]。
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