[单项选择]Every living thing has an inner biological clock that controls behavior. The clock works all the time; even when there are no outside signs to mark the passing of time. The biological clock tells plants when to form flowers and when the flowers should open. It tells insects when to leave the protective cocoon and fly away. And it tells animals when to eat, sleep and wake. It controls body temperature, the release of some hormones and even dreams. These natural daily events are circadian rhythms.
Man has known about them for thousands of years. But the first scientific observation of circadian rhythms was not made until 1729. In that year a French astronomer, Jean Jacques d’Ortous de Mairan, noted that one of his plants opened its leaves at the same time every morning, and closed them at the same time every night. The plant did this even when he kept it in a dark place all the time.
Later scientists wondered about circadian rhythms in humans. They learned that man’s biological
A. every creature has an inner biological clock.
B. the biological clock works day and night.
C. the biological clock has circadian rhythms.
D. the biological clock regulates behavior of creatures.

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[单项选择]Every living thing has an inner biological clock that controls behavior. The clock works all the time even when there are no outside signs to mark the passing of time. The biological clock tells plants when to form flowers and when the flowers should open. It tells insects when to leave the protective cocoon and fly away. And it tells animals when to eat, sleep and wake. It controls body temperature, the release of some hormones and even dreams. These natural daily events are circadian rhythms.
Man has known about them for thousands of years. But the first scientific observation of circadian rhythms was not made until 1729. In that year French astronomer, Jean-Jacques d’Ortous de Mairan, noted that one of his plants opened its leaves at the same time every morning, and closed them at the same time every night. The plant did this even when he kept it in a dark place all the time. Later scientists wondered about circadian rhythms in humans. They learned that man’s biological clock a
A. the regulation of body temperature
B. the secretion of hormones
C. animal reproduction
D. many aspects of plant and animal physiology

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[单项选择]

A Biological Clock

Every living thing has what scientists call a biological clock that controls behavior. The biological clock tells (1) when to form flowers and when the flowers should open. It tells (2) 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 animal (3) the actions of some biological clocks. Scientists recently found, for example, that a tiny animal changes the color of its fur (4) the number of hours of daylight. In the short (5) 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 (6) twice each year. Birds (7) flying become res
A. how
B. why
C. where
D. what

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