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  3. Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to form flowers and

Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to form flowers and

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问题     Every living thing has what scientists call a biological clock that controls behavior. It tells plants when to form flowers and insects when to leave protective cocoon(茧) and fly a-way. And it tells animals and human beings when to awaken, sleep, and seek food. It controls our body temperatures, the release of some hormones(激素)and even dreams.
    Events outside the plant and animal affect its actions. Scientists recently found that a tiny animal called Siberian hamster (大仓鼠) changes the color of its fur because of the num-ber of hours of daylight. In shorter days of winter its fur becomes white. The fur becomes gray-brown in longer hours of daylight in summer.
    Inner signals control other biological clocks. German scientists found that some internal one seems to order birds to begin their migration flights two times each year. Birds prevented from flying become restless when it is time for the trip. But 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 which 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 seems to control the timing of some of our actions. Probably there are other cells to control other body activities. He is studying how they affect the way we do our work. Most of us have great difficulty if we must often change to different work hours. It can take many days for a human body to accept a major change in work hours. Industrial officials should have a better understanding of biological clocks and how they affect workers. He said such an understanding could reduce sickness and accidents at work, and would help increase production.
The author mentions Siberian hamster to illustrate that______.
选项 A、every living thing has a biological clock
B、Siberian hamster is a highly adaptive species
C、biological clocks can change an animal’s skin color
D、external factors can affect the behavior of an organism
答案D
解析 文章第二段的第一句话Events outside the plant and animaI affect its actions．(动植物的外部事件对其行为有影响)是这一段的主题。因此，在这一段中提到的Siberian hamster是为了说明这一主题的。
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