What does the passage mainly discuss?

* Hướng dẫn giải

Đáp án D

Bài văn nói về “Cái gì cấu thành nên ô nhiễm môi trường” với những ý được nêu lên trong bài như: “Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides, are found in nature.” Hay như ở câu cuối của bài: “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.”