单项选择题

During daylight hours, birds ______.

A．wheel back and forth
B．follow the stars in their flights
C．use sun for guidance
D．are quite likely to get lost
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单项选择题
The reason why birds don't get lost on long flights ______.
A．have been known to scientists for many years
B．have only recently been discovered
C．are known by us
D．will probably remain a mystery
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