inwhatwaycannaturalpollutantsplayanimportantrolein-凯发娱乐

in what way can natural pollutants play an important role in controlling air pollution

a．they function as part of a purification process.
b．they are dwarfed by the pollutants produced by human activities.
c．they are less harmful to living beings than are other pollutants.
d．they have existed since the earth developed.

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