单项选择题

About 80% of what you see floating in a ray of sunshine is ______.

A．dust
B．dead human skin
C．quill-coverts
D．fluff
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accents
单项选择题
Why does Catherine say people themselves produce harmful vapors into the indoor air
A．People shed more than any other animal indoors.
B．People do not keep their room tidy.
C．People’s skin flakes are small enough to float in the air and pollute the indoor air.
D．The furniture people buy is in bad quality.
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