单项选择题

A．Both of the activities aren’t very good.
B．He has no interest in doing exercise.
C．They should choose a different activity.
D．It doesn’t matter which activity to choose.
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D
单项选择题
It can be inferred from the passage that oxygen and carbon dioxide ______.
A．produce energy for breathing
B．diffuse immediately in the blood
C．penetrate slowly into the air sacs
D．travel in opposite ways in the lungs
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