单项选择题
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A Special Clock 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______(57) clocks. German scientists found that some kind of internal clock seems’ to order birds to begin their long migration ______(58) twice each year. Birds______(59) from flying become restless when it is time for the trip,______(60) they become calm again when the time of the flight has ended. Scientists say they are beginning to learn which______(61) of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain______(62) to control the timing of some of our actions. These______(63) tell a person when to wake, when to______(64) and when to seek food. Scientists say there probably are other biological clock Cells that ______(65) other body activities. |