2. 外语
  3. Ecology is the science of how living creatures and plants exist together and depend on each other and on the local environment.

Ecology is the science of how living creatures and plants exist together and depend on each other and on the local environment.

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问题     Ecology is the science of how living creatures and plants exist together and depend on each other and on the local environment. Where an environment is undisturbed, the ecology of an area is【C1】________ balance, but if a creature is exterminated (灭绝) or an alien (异类) species【C2】________, then the balance of nature will be【C3】________.
    Man is a part of the【C4】________and has done more to upset the ecology during【C5】________short span on earth than【C6】________other living creature. He has done this by his ignorance, his【C7】________, his thoughtless foolishness and his uncontrolled wastefulness. He has poisoned the【C8】________and polluted both land and water. He has wasted the earth’s natural resources with no【C9】________for the future, and has thought out the most devastating ways of killing his fellow men and every other sort of life at the same time.
    Since man has done so much【C10】________, it is【C11】________to man to try to put matters right—if it is not already too【C12】________.If there is to be any remedy for our ills, that remedy ultimately lies in the hands of the【C13】________, and the sooner they start doing something about it, the【C14】________.
    One of the main【C15】________of the earth’s troubles is that the world is overpopulated and that this overpopulation is growing【C16】________an ever-increasing rate. At the same time we are【C17】________up our natural resources—fuels and mineral ores—at an ever-increasing speed with no hope of replacing them. For many years the earth has been unable to【C18】________enough food for these rapidly expanding populations and the position is steadily deteriorating【C19】________the fertility of some of our richest soils has been lost and vast areas that were【C20】________fertile lands have turned into barren deserts.
【C20】
选项 A、ever
B、once
C、since
答案B
解析 onee意为“曾经，一度”，ever的准确含义是“在任何时候，从来”。since意为“自从”。本句说的是曾经大片肥沃的土地变成了贫瘠的沙漠。所以once符合语境，故选B。
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