What does the passage mainly discuss?

Câu hỏi: Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the questions.
A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals vegetations, or materials adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled-a far cry from the extensive list of harmful substances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions.
Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides, are found in nature. As the Earth developed, the concentrations of these pollutants were altered by various chemical reactions, they became components in biogeochemical cycle. These serve as an ai purifiation scheme by allowing the compounds to move from the air to the water or soil on a global basis nature's output of these compounds dwarfs that resulting from human activities.
However, human production usually occurs in a localized area, such as a city. In this localized regions human output may be dominant and may temporarily overload the natural purifiation scheme of the cycle The result is an increased concentration of 20% noxious chemicals in the air. The concentrations at which the adverse effects appear will be greater than the concentrations that the pollutants would have in the absence of human activities. 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. 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.
ppm and is not usually a pollutant until its level reaches about 15 ppm.
What does the passage mainly discuss?
A. The effects of compounds added to the atmosphere
B. The economic impact of air pollution
C. How much harm air pollutants can cause
D. What constitutes an air pollutant