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
Diffusion of light in the sky occurs owing to ______.
A．atmospheric impurities
B．molecules of gas
C．varied shades of colour
D．different times of the day
单项选择题
When sunlight enters a dark room through the blinds ______.
A．particles of dust are suspended
B．tiny dust particles become visible
C．it is very bright
D．it becomes visible
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