Narrator Listen to part of a talk in an environmental science class. Now get ready to answer the questions. You may use

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问题     Narrator
    Listen to part of a talk in an environmental science class.
    Now get ready to answer the questions. You may use your notes to help you answer.
According to the speaker, what is the feature of air pollutants?
How much can you say about air pollutant? At first let’s have a look at a useful definition of it. A pollutant is a compound added directly or indirectly by humans to the atmosphere, and in such quantities to adversely affect humans, animals, vegetations or materials. On the other hand air pollution has a very flexible definition that allows continuous change. When the first air pollution law was established in England in the 14th century, air pollutants were limited to compounds that could be seen or smelled, which is very different from the list of harmful substances known today. As technology has developed and knowledge of health aspects has increased, the list of air pollutants has lengthened. In the future even water vapor might be thought of as an air pollutant under certain conditions.
    Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, and nitrogen oxides are found in nature. Perhaps in your daily life you can’t feel them clearly since they don’t make up a big percentage of the air. Before we have learnt something about the properties of the above substances, let’s have a look at their historical development. As the earth developed, the concentrations of these pollutants were altered by various chemical reactions and become biogeochemical components, and then they 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 those resulting from human activities. However, human production usually occurs in a localized area, such as a city.
    In these localized regions, human output may be dominant and may temporarily overload the natural purification scheme of cycles. The result is an increased concentration of human activities. The actual concentration need not be large for a substance to be a pollutant; in fact the concentration 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, often shortened to 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.

选项 A、Its contents often remain stable.
B、Its definition will change over time.
C、Air pollutants are natural.
D、Air pollutants are man-mad

答案B

解析 本题为细节题,考查考生抓住文章中所谈到的重要细节或事实的能力。题目问:说话者认为什么是空气污染物的主要特征?文章的第一段提到“空气污染的定义很灵活”,“随着技术发展和关于健康知识的增加,空气污染物的种类也在增多”,此外,“空气污染物有些是自然界中固有的,有些是人为产生的”,因此综合上述,选项B为正确答案,选项A、C、D都不对。
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