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单项选择题

The reason why birds don't get lost on long flight ______.

A．has been discovered recently
B．is known by everybody
C．still remains a mystery
D．has been known to scientists for years

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单项选择题Which of the following is referred to as the Dark Ages

A．The year 1600.
B．The 1600s.
C．The time between 1606 and 1871.
D．The period from the 4th to the 14th century.

单项选择题Par

A．2 mainly explains ______.A. what damage was caused by ChicagoB. how the bakers were carelessC. why so many fires happened in the 1600sD. which cities had fare brigades in the 1600s