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
Which of the following statements about human alveolar macrophage is TRUE
A．Airborne dust may trigger suppressor macrophage suicides.
B．Airborne dust may trigger stimulatory macrophage suicides.
C．The ratio of suppressor macrophages to stimulatory ones in people with lung diseases is much higher than that in people with healthy lungs.
D．Only stimulatory macrophages show responses to airborne dust.
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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