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提问人：网友xjz36998494
发布时间：2022-01-07
[主观题]
What does the professor mean when he says this？A．Little is known about defining a substanc
What does the professor mean when he says this？
A．Little is known about defining a substance as an air pollutant.
B．The substance isn"t an air pollutant when there is a very little in the air.
C．The substance can"t be judged to be an air pollutant only from its actual concentration level.
D．The actual concentration level means little to people.
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