Question 46: 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. According to the passage, the numerical value of the concentration level of a substance is only useful if .
Key words: numerical value, concentration level.
Question: According to the passage, when is the numerical value of the concentration of a substance useful only?
Clue: “…the numerical value tells us little until 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” Analysis: Immediately after mentioning the numerical value, the author gives an example in which there are The occurrence of two variables is “concentration level” and “natural level”. Therefore, for “concentration level” to be meaningful, it is also necessary to know “natural level”. Choose answer C. the natural level is also known.
The other answers are not suitable:
the other substances in the area are known: other substances in nature that are known.
it is in a localized area: substances present in the area of specialization.
it can be calculated quickly: it must be calculated quickly.