单项选择题

案例分析题 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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单项选择题
The experiment with the dove implied that ______.
A．birds should be taught to navigate
B．a dove that has been caged will not fly long distances
C．some birds cannot fly at night
D．some birds seem to follow the stars when they fly at night
单项选择题
In the paragraph 3, his outdoor cousins means ______.
A．other relatives
B．the other doves of the same brood (n. 窝)
C．doves under the natural sky
D．other birds in general
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