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 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 Siberian 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 in 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 the 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.【C16】______ can take many days for a human body to【C17】______ a 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、may
B、can
C、must
D、might
答案C
解析 考查情态动词 。情态动词的使用应该参照语境。人们更换工作时间一般是迫不得已而为之，因此只有等到必须做的时候才会更换。must表示“必须”，所以C项正确。
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