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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 all 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 ascientific 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 them extends them again, 【C17】______ he 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 thousand fold.
【C19】
选项 A、unless
B、when
C、until
D、after
答案C
解析 根据固定结构搭配：not．．．until．．．，正确答案是C。
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