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
You had better go there by train, ______.
A．don't you
B．won't you
C．hadn't you
D．aren't you
单项选择题
This factory can ______ 10,000 bikes every year.
A．turn out
B．turn up
C．turn in
D．turn over
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