单项选择题

The reason why birds don't get lost on long flight ______.

A．has been discovered recently
B．is known by everybody
C．still remains a mystery
D．has been known to scientists for years
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单项选择题
Under man-made stars, the bird in the cage ______.
A．tried to fly in the opposite direction of birds not caged
B．changed direction when the position of the stars was changed
C．wouldn't fly well
D．stayed where they were
单项选择题
In total darkness, doves ______.
A．circle helplessly
B．use landmarks
C．find their way by clouds
D．fly back home
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