Questions 16 to 20 are based on the following passage:
　　In these days of technological triumphs, it is well to remind ourselves from time to time that living mechanisms are often incomparably more efficient than their artificial imitations. There is no better illustration of this idea than the sonar (声纳) system of bats. It is billions of times more efficient and more sensitive than the radars and sonars designed by man.
　　注：1.triumph 胜利，成功
　　2.it is well to 有必要
　　3.incomparably 不可比地
　　4.第一句主题句：天然胜于人工 artificial 人工的5.illustration 例证，本段末句强烈结论
　　Of course, the bats have had some 50 million years of evolution to refine their sonar. Their physiological mechanisms for echo location, based on all this accumulated experience, therefore deserve our thorough study and analysis.
　　注：1.refine 提炼、改善
　　2.二段首句是对首段末句的解释
　　3.50 million years对应的是第17小题
　　17. The author suggests that the sonar system of bats ____
　　A) was at the height of its perfection 50 million years ago
　　B) is better than man-made sonar because it has had 50 million years to be be refined
　　C) should have been discovered by man many years ago
　　D) is the same as it was 50 million years ago
　　注：B 选项体现一段结尾和二段开头，应该优选，含有关系词 because,although,not only...but also, as well as 的应该优选
　　To appreciate the precision of the bats' echo location, we must first consider the degree of their reliance upon it. Thanks to sonar, an insect-eating bat can get along perfectly well without eyesight. This was brilliantly demonstrated by the Italian naturalist Lazzaro Spallanzani. He caught some bats in a bell tower, blinded them and released them outdoors. Four of these blind bats were recaptured after they had found their ways back to the bell tower, and on examining their stomachs' contents, Spallanxani found that they had been able to capture and eat flying insects. We know from experiments that bats easily find insects in the dark of night, even when the insects make no sound that can b

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