单项选择题

By "his outdoor cousins" the author means ______.

A．other experimenters
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
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
单项选择题
The main idea of the third paragraph is stated in the ______.
A．first sentence
B．second sentence
C．third sentence
D．last sentence
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