Question 49: 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: An air pollutnt is defined as a compound added directly or questions by humans to the In such quantities as to affect atmosphere, animals, vegetation, or materials affecting. 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 subtances 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, carbon monoxide, and notrigen 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 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 results from human activities. However, human production usually occurs in a localized area, such as as city. In such a 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 subtance to be a pollutant; in fact, the numerical value tells us little we know how much of an increase this represents over the concentration that would naturally occur 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. The word “These” in the second paragraph is closet in meaning to .
Key words: these, second paragraph, closet in meaning.
Clue: “…they became components in biogeochemical cycles. These serve as an air purification scheme by…”: … they become part of the biogeographic cycle. These substances act as an air purification program by…
Analysis: “They” here is the noun phrase mentioned just before. So choose answer D. the components in biogeochemical cycles.