2. 外语
  3. Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to【C1】______flowers

Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to【C1】______flowers

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问题     Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to【C1】______flowers and insects when to leave the protective cocoon (茧) and fly away. And it tells animals and human beings when to【C2】______. sleep and seek food. It controls our body temperatures, the【C3】______of some hormones and even dreams.
    Events【C4】______the plant and animal affect its actions. Scientists recently found that a tiny animal called Siberain hamster changes the color of its【C5】______because of the number of hours of【C6】______. In shorter days of winter its fur becomes white. The fur becomes gray-brown in longer【C7】______of daylight summer.
    【C8】______signals control other biological clocks. German scientists found that some internal one seems to order birds to begin their【C9】______flights two times each year. Birds prevented from flying become【C10】______when it is time for the trip.【C11】______they become calm again when the time of the flight has ended. A mix of outside and internal events controls some biological clocks; such things as heartbeat and the daily change from sleep to walking take place because of both external and internal signals.
    Scientists are beginning to learn【C12】______parts of the brain contain biological clocks. A researcher at Harvard University, Dr. Martin Moore Ede, said a small group of cells near the front of the brain seemed to control the【C13】______of some of our actions. Probably there are other cells to control other body activities. He is studying【C14】______they affect the way we do our work. Most of us have great difficulty if we【C15】______often change to different work hours. It can【C16】______many days for a human body to【C17】______the major change in work hours. Industrial officials should have a better【C18】______of biological clocks and how they affect workers. He said such an understanding could【C19】______sickness and accidents at work, and would help increase【C20】______.
【C15】
选项 A、can
B、must
C、might
D、may
答案B
解析 语义衔接题。人们更换工作时间一般是迫不得已而为之，只有[B]must有这层含义，故为答案。
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