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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单项选择题
Which of the following best describes the function of the third paragraph of the passage
A．It presents a specific example of the process discussed in the first two paragraphs.
B．It explains a problem that the researchers mentioned in the second paragraph has yet to solve.
C．It evaluates the research discoveries described in the previous paragraph.
D．It begins a detailed description of the process introduced in the previous two paragraphs.
填空题
after→before
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