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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

admin2010-01-18  68
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.
We can learn from the passage that the tired land has fully recovered ______.
选项 A、when grasses and other plants appear again
B、when the treatment is given by nature alone
C、after it has been lying idle for ten years
D、after grasses and other plants have decayed again
答案D
解析 根据文章第二段中的“The tired land then slowly recovered. Gradually grasses and other plants would appear on it and its productive power would be slowly by their decay .”则可知[D]正确。
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