单项选择题

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
______ in India, the banana was brought to the America by the Portuguese who found it in Afric
A． Originally cultivatedB. Originally being cultivatedC. Having originally cultivatedD. Although it originally cultivated
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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