单项选择题

According to this passage, Environmental Protection Agency ______.

A．is the only agency studying the effects of air pollution on health
B．has launched a surprised attack on researchers
C．has decided to decrease the permitted concentration of airborne dust
D．has tremendously improved pollution control
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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
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
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