According to the passage, the numerical value of the concentration level

* Hướng dẫn giải

Đáp án D

Thông tin ở đoạn cuối: “The actual concentration need not be large for a substance to be a pollutant; in fact, the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area.” (=the numerical value of the concentration level of a substance is only useful if the natural level is also known)