2. 外语
  3. An animal species becomes extinct when it fails to produce enough young in each generation to keep pace with the death-rate. We

An animal species becomes extinct when it fails to produce enough young in each generation to keep pace with the death-rate. We

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问题    An animal species becomes extinct when it fails to produce enough young in each generation to keep pace with the death-rate. We can【C1】______from fossil evidence in rocks that many living species have become extinct over the millions of years【C2】______life began. It is a natural【C3】______and extinction is the【C4】______of any animal that has specialized too far to change【C5】______its environment changes, or has to compete with a better-【C6】______and more powerful animal.【C7】______remarkable technical developments during the past few centuries, man has destroyed or【C8】______destroyed some species by killing them【C9】______such a rate that they couldn’t produce enough offspring, or by completely changing their natural environment at surprising speed.
   A number of examples can be given of the way【C10】______which natural environments are being rapidly changed—Amazonia, for instance. There is every【C11】______that many species of animals will be made extinct because of these and similar【C12】______of natural vegetation. Large numbers of animals have been hunted and killed for food. The North American buffalo is a【C13】______of the near-extinction of a species through hunting. Often the numbers are【C14】______great that the hunters may not realize the danger. But even when the danger is widely【C15】______, the financial rewards for the hunters may be so great that they choose to【C16】______the threat to the species. Attitudes like this have【C17】______to hunters killing animals for furs, for ivory or merely for ornaments. A slight【C18】______on this is when tourists hunt animals for trophies. Such magnificent creatures【C19】______lions and tigers【C20】______out of existence in some parts of the world.  
【C5】
选项 A、because
B、if
C、unless
D、when
答案D
解析 由常识动物不能随环境变化而变化就会灭绝，所以用when表示“当…时候”最为恰当。
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