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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(有害地). 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(悬殊的差别)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(水蒸气)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(浓度)of these pollutants were altered by various chemical reactions; they became components in biogeochemical(生物地球化学的)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, 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(有害的)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 0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
Which of the following is best supported by the passage?
选项 A、To effectively control pollution, local government should regularly revise the air pollution laws.
B、One of the most important steps in preserving natural lands is to better enforce air pollution laws.
C、Scientists should be consulted in order to establish uniform limits for all air pollutants.
D、Human activities have great impact on air pollution.
答案D
解析 A项是“为有效控制污染，地方政府必须定期修订空气污染法规”，B项是“保护自然土地最重要的一步就是更好地强制推行空气污染的相关法规”，文中没有谈到与法规有关的事项，故二者都不能入选。C项是“为了设定针对所有空气污染物的统一限制，需咨询科学家的意见”在文中也未提及。D项是“在减轻空气污染方面，人类活动将具有影响力”，原文提到“人类产生的空气污染物可能会占据主导地位。而且还有可能暂时超出生物地球化学循环的自然净化能力。这种情况导致的后果就是空气中不断汇集有害的化学物质。”所以如果人类的活动有所调整，将对减轻空气污染有积极的影响。故选D。
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