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

* Hướng dẫn giải

Đáp án : C

Ta có các thông tin sau:

“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”

“For example, sulfur dioxide has detectable health effects at 0.08 parts per million (ppm), which is about 400 times its natural level. Carbon monoxide, however, has a natural level of 0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm” => C