An air pollutant is defined as a compound added directly the word "these" in the second

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Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the question from 1 to lớn 7

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An air pollutant is defined as a compound added directly or indirectly by humans to lớn the atmosphere in such quantities as lớn 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 Englvà in the fourteenth century, air pollutants were limited to lớn compounds that could be seen or smelled - a far cry from the extensive list of harmful substances known today. As technology has developed & 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, carbon monoxide, và nitrogene oxides, are found in nature. As the Earth developed, the concentration of these pollutants was altered by various chemical reactions; they became components in biogeochemical cycles. These serve sầu as an air purification scheme by allowing the compounds khổng lồ move from the air lớn 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 đô thị. In such a region, human output may be dominant & 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 sầu 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 màn chơi. Carbon monoxide, however, has a natural level of 0.1 ppm & is not usually a pollutant until its màn chơi reaches about 15 ppm.