According to the passage, the numerical value of the concentration level of a substance is only useful if ______.

A. the other substances in the area are known  

B. it can be calculated quickly.

As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil.

Chọn D