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【单选题】Biological ClockWhen the sun rises on a warm and sunny day, you can see some flowers open up. When they close late in the afternoon, other flowers open.Biological rhythms（生物节律）, like the opening and c...
A.
But where can you find it?
B.
This makes them feel tired.
C.
In the summer they fly back to have babies.
D.
We are learning more and more about them.
E.
Biological clocks can be adjusted.
F.
Other rhythms are based on months, seasons or years.