单项选择题

This passage is mainly about ______.

A．how inhaled dust harms the lungs
B．the function of Environmental Protection Agency
C．the function of human alveolar macrophages
D．studies by Environmental Protection Agency
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单项选择题
It is implied in the passage that ______.
A．many industrialists feel no concern for the improvement of pollution control
B．breathing low concentrations of urban dust does not necessarily sicken or kill people
C．no acceptable biological mechanism has been suggested to explain the toxicity of low concentration of urban dust
D．scientists have done nothing to reveal the mystery of the toxicity of low concentration of urban dust
单项选择题
How did the nineteenth-century defenders of photography stress the photography
A．They stressed photography was a means of making people happy.
B．It was art for recording the world.
C．It was a device for observing the world impartially.
D．It was an art comparable to paintin
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