单项选择题

Exhaled cigarette smoke is yellowish because ______.

A．it comes through the mouth
B．it contains nicotine
C．of water vapour in the mouth
D．it is a good diffuse of light
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单项选择题
The sky has a darker blue colour when we look at it ______.
A．near a mountain
B．against a mountain
C．from the top of a mountain
D．at the foot of a mountain
单项选择题
The colour of the sky at the horizon depends on ______.
A．the amount of light
B．the condition of the atmosphere
C．the time of the day
D．the thickness of air
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