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 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

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