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  3. A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantitie

A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantitie

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问题     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, vegetation, or material 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 vapour might be considered an air pollutant under certain conditions.
    Many of the more important air pollutants, such as sulphur 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 cycles. These reactions serve as an air purification 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 region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of 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, sulphur 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.
What effect did the development of earth have on the air pollutants such as carbon monoxide?
选项 A、Their harm to human activities became more serious.
B、Their ability to purify air was improved a lot.
C、Their concentrations were changed by chemical reactions.
D、Their quantities in the air were mainly from human activities.
答案C
解析 细节题。根据carbon monoxide可定位到第二段As the earth developed，theconcentrations of these pollutants were altered by various chemical reactions；theybecame components in biogeochemical cycles．These serve as an air purification schemeby allowing the compounds to move from the air to the water or soil．“随着地球的发展，这些污染物的浓度会随着各种化学反应而发生改变：他们成为了生物地球化学循环的组成部分。这些循环使得这些化合物从空气中转移到水中或土壤中，从而达到净化空气的作用。”和C选项内容一致，其中changed和原文altered为同义转换，所以答案选C。
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