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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单项选择题
The experiment with the dove indicated that ______.
A．birds have to be taught to navigate
B．a bird 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
Most students ______ to the Summer Camp were good students.
A．inviting
B．invite
C．to invite
D．invited
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