单项选择题

By "his outdoor cousins" the author means ______.

A．other experimenters
B．B ) the other doves of the same brood
C．doves under the natural sky
D．other birds in general
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单项选择题
During daylight hours, birds ______.
A．fly aimlessly
B．rely on landmarks
C．use sun for guidance
D．are more likely to get lost
单项选择题
The reason why birds don't get lost on long flights ______.
A．have been known to scientists for years
B．have only recently been discovered
C．are known by everyone
D．will probably remain a mystery
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