单项选择题

The second paragraph is mainly about ______.

A．the wheel in transportation
B．the roller in transportation
C．animals in transportation
D．very heavy loads
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单项选择题
By his outdoor cousins the author means ______.
A．other experimenters
B．B ) the other doves of the same brood
C．doves under the natural sky
D．other birds in general
单项选择题
During daylight hours, birds ______.
A．fly aimlessly
B．rely on landmarks
C．use sun for guidance
D．are more likely to get lost
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