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正确答案：In this lecture, the professor probes into two different kinds of factors that determine the animals' biological cycles, internal clocks and external clocks, which can affect each other. The professor takes flying squirrel as an example. In the experiment, when flying squirrels are kept in constant darks for about a month, they are still able to keep regular cycles of activity，which shows that internal clocks do work in flying squirrels when the external environment has changed, although it is about half an hour shorter than the normal length of time. On the other hand. When these squirrels are exposed to the daylight again after a month of darkness, they are not as active as they usually are during the nighttime and wake up during the day. And then, the external clocks, along with the cycle of light and darkness, begin to adjust the squirrels’ internal clocks. Finally, their biological cycles return to normal again.
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