{{B}}A Special Clock{{/B}} Every living thing has what scientists call a biological clock that controls behavior. The biologicai clock______(51) plants when to form flowers and when the flowers should open. It tells insects when to______(52) the protective cocoon (防护卵袋) and fly away, and it tells animals and human beings when to eat, sleep and wake. Events outside the plant and animal______(53) the actions of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur because of the______(54) of hours of daylight. In the short______(55) of winter, its fur becomes white. The fur becomes gray brown in______(56) in the longer hours of daylight in summer. Inner signals control other______ A. unless B. but C. therefore D. if 更多"阅读下面的短文,文中有15处空白,每处空白给出了4个选项,请根据短文的"的相关试题: [单项选择] 阅读下面的短文,文中有15处空白,每处空白给4个选项,请根据短文的内容从4个选项中选择1个最佳答案。
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