单选题 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 0.1ppm and is not usually a pollutant until its level reaches about 15ppm.

单选题 It can be inferred from the first paragraph that ______.

单选题 What effect did the development of earth have on the air pollutants such as carbon monoxide?

【正确答案】 C

【答案解析】细节题。根据carbon monoxide可定位到第二段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. “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。

单选题 According to the passage, what can we know about human-generated air pollution in cities?

【正确答案】 B

【答案解析】细节题。根据human-generated air pollution将答案定位到第三段In this localized region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles.“在这些局部区域，污染物可能主要来自人类活动，而且可能会暂时超出自然循环的净化能力。”可知选项B是对原文的同义转述，其中overburden和原文的overload互为近义词。所以答案选B。

单选题 The author puts forward the examples of sulphur dioxide and carbon monoxide at last to show that ______.

【正确答案】 C

【答案解析】细节题。根据原文第三段最后三句“实际上，浓度值并不能告诉我们什么，除非我们能知道这个浓度值相对于这种物质在该地区自然状态下的浓度的增长幅度。比如，当每百万个气体分子中含有0.08个二氧化硫气体分子时，人体就会明显感到不适。这个浓度大约是正常浓度的400倍。而一氧化碳的正常浓度是百万分之零点一，在不超过百万分之十五时就不算污染物。”可知化学物质的浓度值不是界定污染物的关键，和选项C的表述一致。所以答案选C。