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 main idea of the third paragraph is stated in the ______.
A．first sentence
B．second sentence
C．third sentence
D．last sentence
单项选择题
The best title for the passage is ______.
A．"Traveling Becomes Easier"
B．"The Invention of the Wheel"
C．"The Stagecoach in America"
D．"The Importance of Wheel"
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