Questions下列各 are based onthe following passage. A useful definition of an air polluta

ntis a compound added directly or indirectly by humans to the atmospherein suchquantities as to affect humans, animals, vegetation, or material adversely (有害地．. Air pollution requires avery flexibledefinition that permits continuous change. When the first air pollution lawswere established inEngland in the fourteenth century, air pollutants werelimited to compounds that could be seen or smelled—a farcry (悬殊的差别． from the extensive list of harmful substances known today. Astechnology has developed andknowledge of the health aspects of variouschemicals has increased, the list of air pollutants has lengthened. Inthefuture, even water vapor (水蒸气． might be consideredan air pollutant under certain conditions. Many of the more important airpollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides,arefound in nature. As the Earth developed, the concentrations (浓度． of these pollutants were altered byvariouschemical reactions; they became components in biogeochemical (生物地球化学的． cycles. These serve as an airpurification scheme by allowing thecompounds to move from the air to the water or soil. On a global basis,naturesoutput of these compounds dwarfs that resulting from human activities. However,human productionusually occurs in a localized area, such as a city. In this localized region, human outputmay be dominant and may temporarily overload the natural purificationscheme ofthe cycles. The result is an increased concentration of noxious (有害的． chemicals in the air. Theconcentrations atwhich the adverse effects appear will be greater than the concentrations thatthe pollutants wouldhave in the absence of human activities. The actualconcentration need not be large for a substance to be apollutant; in fact thenumerical value tells us little until we know how much of an increase thisrepresents over the concentration that would occur naturally in the area. Forexample, sulfur dioxide has detectable health effects at0.08 parts per million(ppm．, which is about 400 times its natural level.Carbon monoxide, however, has a naturallevel of 0.1 ppm and is not usually apollutant until its level reaches about 15 ppm. It can be inferred from thefirst paragraph that _________

A.water vapor is an air pollutant in localized areas

B.the definition of air pollution will continue to change

C.a substance becomes an air pollutant only in cities

D.most air pollutants today can be seen or smelled