A. They function as part of a purification process.
B. They occur in greater quantities than other pollutants.
C. They are less harmful to living beings than other pollutants
D. They have existed since the Earth developed
Đáp án : A
“Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides, are found in nature”
“These serve as an air purification scheme…” : những cái này đóng vai trò như 1 quy trình làm sạch => A
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