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
Experiments showed that ______.
A．birds are likely to get lost if there aren't man-made stars.
B．day-flying birds depend on the sun while night-flying birds depend on the stars
C．birds depend on the sun to guide them
D．birds depend on the stars to guide them
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