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  3. The ability of falling cats to right themselves in midair and land on their feet has been a source of wonder for ages. Biologist

The ability of falling cats to right themselves in midair and land on their feet has been a source of wonder for ages. Biologist

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问题      The ability of falling cats to right themselves in midair and land on their feet has been a source of wonder for ages. Biologists long regarded it as an example of 【C1】______  by natural selection, but for physicists it bordered on the miraculous. Newton’s laws of motion 【C2】______  that the total amount of spin of a body cannot change 【C3】______  an  external torque speeds it up or slows it down. If a cat has no spin when it is released and experiences no external torque, it ought not to be able to 【C4】______  as it falls.
     In the speed of its execution, the  【C5】______  of a tumbling cat resembles a magician’s trick. The gyrations of the cat in midair are 【C6】______  fast for the human eye to follow, so the process is obscured.【C7】______  the eye must be speeded up, or the cat’s fall slowed down for the phenomenon to be observed.  A century ago the former was accomplished 【C8】______  high-speed photography using equipment now  【C9】______  in any pharmacy. But in the nineteenth century the 【C10】______  on film of a falling cat constituted a scientific experiment.
     The experiment was described in a paper presented to the Paris Academy in 1894. Two sequences of twenty photographs【C11】______  one from the side and one from behind, 【C12】______ a white cat in the act of righting itself. Grainy and quaint  【C13】______  they are, the photos show that the cat was dropped upside down, with no initial spin, and still landed on its feet. Careful analysis of the photos reveals the secret. As the Cat rotates the front of its body 【C14】______ , the rear and tail twist counterclockwise,  【C15】______  the total spin remains zero, 【C16】______  Newton’s laws. Halfway down, the cat pulls in its legs before reversing its twist and then extends them again, 【C17】______  the desired end result. The explanation was that while no body can acquire spin without torque, a flexible one can readily change its 【C18】______  . Cats know this instinctively, but scientists could not be sure how it happened 【C19】______ they increased the speed of their 【C20】______  a thousandfold.
【C12】
选项 A、showed
B、shows
C、show
D、had shown
答案C
解析 照片虽然摄于1894年，但现在仍存在，因此用现在时，而且主语是复数，故选C。
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