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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 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(硫化物), 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 serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, natures 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 effect appears 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 natural level of 0. 1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
From the first paragraph, we can infer that______.
选项 A、water vapor is an air pollutant in localized areas
B、today most air pollutants can be seen or smelled
C、the definition of air pollution will continue to change
D、a substance becomes an air pollutant only in cities
答案C
解析 由题干提示定位至第一段。根据该段第二句可知，C)“空气污染的定义会继续改变”与此表述一致，而且在本文中也详细说明了这一变化情况，故为正确答案。
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