【答案解析】[听力原文] 24-29 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. [解析] 本题为要点题，要求考生必须具备综合概括文章主要信息的能力。题目问的是全文的中心思想，综合原文不难看出话题是围绕空气污染物的定义而展开的，而不是它的原因、危害性和预防方法，由此可见A项应为正确答案。
多选题
According to the speaker, what is the feature of air pollutants?
多选题
According to the speaker, what might be an air pollutant in the future?
A. Sulfur oxides.
B. Carbon monoxide.
C. Nitrogen oxides.
D. Water vapor.
【正确答案】
【答案解析】[解析] 本题为细节题。题目问：说话者认为将来什么可能是空气污染物?文章的第一段指出“In the future even water vapor might be thought of as an air pollutant under certain conditions”，也就是说，水蒸气现在并不是大气污染物，将来可能成为污染物。其他几项均为干扰项，因为它们现在已经是污染物了，因此我们可以得出选项D为正确答案。
多选题
What does the professor mean when he says this?
A. Little is known about defining a substance as an air pollutant.
B. The substance isn't an air pollutant when there is a very little in the air.
C. The substance can't be judged to be an air pollutant only from its actual concentration level.
D. The actual concentration level means little to peopl
【正确答案】
【答案解析】[解析] 本题为语用理解题，考查考生是否能听懂说话者说话的意图。题目问：教授说这一段话是什么意思?原文第三段提到“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”，这段话的大意是：只有知道这种物质的实际含量比空气中的自然含量增加了多少才能断定它是不是污染物，因此C项为正确答案。
多选题
What can be inferred from this?
A. Sometimes natural air pollutants can purify air.