2. 外语
  3. It has been known since ancient times that certain plants regularly open their leaves in daytime and close them at night. It was

It has been known since ancient times that certain plants regularly open their leaves in daytime and close them at night. It was

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问题     It has been known since ancient times that certain plants regularly open
their leaves in daytime and close them at night. It was assumed which this        【S1】______.
periodic change was triggered by the alternating sunlight and darkness. Such
leaves stick to their daily routine even when keeping in constant darkness.       【S2】______.
    What is circadian rhythm? Many biological processes follow a 24-hours         【S3】______.
schedule. These cycles are called circadian rhythms. These innate rhythms are
governed by sophisticated chemical mechanisms can be described as biological      【S4】______.
clocks. Such rhythms are important for humans too! We sleep through most
of the hours of darkness and are awaken through most of the hours of              【S5】______.
daylight. Body temperature increases during the day and decreases during the
night. The secretion of various hormones that controls life’s processes rises     【S6】______.
and falls to a circadian rhythm.
    How does this relate to health? Circadian rhythms are important for
desirable functioning of the processes vital to health. This is most effectively
illustrated and the normal circadian rhythms are disrupted by lack of sleep or by  【S7】______.
crossing time zones. It may take days or even hours for all the bodies circadian   【S8】______.
rhythms to read just appropriately. The turmoil (混乱) that results from the
readjustment process when time zones are crossed experienced as the                【S9】______.
phenomenon of jet lag. This is also experienced by people who do shift work
Understanding circadian rhythms may be irrelevant to the understanding and         【S10】______.
treatment of disease. Therapy can be more effective and toxic effects reduced
when drugs are administered at carefully selected times of the day.
【S1】
选项
答案which→that
解析 从语法结构看，句中的it 是形式主语，真正的主语是assumed 后边的主语从句。而该主语从句中不缺任何成分，所以应该用that 引导。
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