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  3. A Biological Clock Every living thing has what scientists call a biological clock that controls behavior. The biological clo

A Biological Clock Every living thing has what scientists call a biological clock that controls behavior. The biological clo

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问题                             A Biological Clock
    Every living thing has what scientists call a biological clock that controls behavior. The biological clock tells【C1】______when to form flowers and when the flowers should open. It tells【C2】______when to leave the protective cocoons and fly away, and it tells animals and human beings when to eat, sleep and wake.
    Events outside the plant and anima【C3】______the actions of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur【C4】______ the number of hours of daylight. In the short【C5】______of winter, its fur becomes white. The fur becomes gray brown in color in the longer hours of daylight in summer.
    Inner signals control other biological clocks. German scientists found that some kind of internal clock seems to order birds to begin their long migration【C6】______twice each year. Birds【C7】______flying become restless when it is time for the trip,【C8】______they become calm again when the time of the flight has ended.
    Scientists say they are beginning to learn which【C9】______of the brain contain biological clocks. An American researcher, Martin Moorhead, said a small group of cells near the front of the brain【C10】______to control the timing of some of our actions. These【C11】______tell a person when to【C12】______, when to sleep and when to seek food. Scientists say there probably are other biological clock cells that control other body activities.
    Dr. Moorhead is studying【C13】______our biological clocks affect the way we do our work. For example, most of us have great difficulty if we must often change to different work hours.
    【C14】______can take many days for a human body to accept the major change in work hours. Dr. Moorhead said industrial officials should have a better understanding of biological clocks and how they affect workers. He said【C15】______understanding could cut sickness and accidents at work and would help increase a factory’s production.
【C8】
选项 A、and
B、but
C、therefore
D、however
答案B
解析 很明显，这里需使用连词，连接该句中平行的两个分句。而根据整个句子的句意，上半句和下半句形成反差对比：become restless when the time of the flight has ended，所以应选择but，表示相反的两层意思。
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