📖 原文
(1) A useful definition of an air pollutant is a compound added directly or indirectly by (2) humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or (3) materials adversely. Air pollution requires a very flexible definition that permits (4) continuous change. When the first air pollution laws were established in England in the (5) fourteenth century, air pollutants were limited to compounds that could be seen or (6) smelled — a far cry from the extensive list of harmful substances known today. As (7) technology has developed and knowledge of the health aspects of various chemicals has (8) increased, the list of air pollutants has lengthened. In the future, even water vapor (9) might be considered an air pollutant under certain conditions. (10) Many of the more important air pollutants, such as sulfur oxides, carbon monoxide, (11) and nitrogen oxides, are found in nature. As the Earth developed, the concentrations of (12) these pollutants were altered by various chemical reactions; they became components in (13) biogeochemical cycles. These serve as an air purification scheme by allowing the (14) compounds to move from the air to the water or soil. On a global basis, nature's output of (15) these compounds dwarfs that resulting from human activities. However, human production (16) usually occurs in a localized area, such as a city. In this localized region, human (17) output may be dominant and may temporarily overload the natural purification scheme (18) of the cycles. The result is an increased concentration of noxious chemicals in the air. (19) The concentrations at which the adverse effects appear will be greater than the (20) concentrations that the pollutants would have in the absence of human activities. The (21) actual concentration need not be large for a substance to be a pollutant; in fact the (22) numerical value tells us little until we know how much of an increase this represents (23) over the concentration that would occur naturally in the area. For example, sulfur (24) dioxide has detectable health effects at 0.08 parts per million (ppm), which is about 400 (25) times its natural level. Carbon monoxide, however, has a natural level of 0.1 ppm and is (26) not usually a pollutant until its level reaches about 15 ppm.
❓ 试题解析
问题 41： What does the passage mainly discuss?
A. The economic impact of air pollution
B. What constitutes an air pollutant
C. How much harm air pollutants can cause
D. The effects of compounds added to the atmosphere
✅ 正确答案： B）What constitutes an air pollutant
📝 解析： 全文讨论空气污染物的定义、自然来源与人为来源、浓度等。
💡 解题技巧： 主旨题看首段的定义性陈述。
问题 42： The word "adversely" in line 3 is closest in meaning to
A. negatively
B. quickly
C. admittedly
D. considerably
✅ 正确答案： A）negatively
📝 解析： "affect ... adversely" — adversely = 不利地/负面地。
💡 解题技巧： 词汇题：adversely = negatively = harmfully。
问题 43： It can be inferred from the first paragraph that
A. water vapor is an air pollutant in localized areas
B. most air pollutants today can be seen or smelled
C. the definition of air pollution will continue to change
D. a substance becomes an air pollutant only in cities
✅ 正确答案： C）the definition of air pollution will continue to change
📝 解析： 首段说定义需要灵活、允许持续变化，将来水蒸气也可能被视为污染物。
💡 解题技巧： 推理题定位"flexible definition that permits continuous change"。
问题 44： The word "altered" in line 12 is closest in meaning to
A. eliminated
B. caused
C. slowed
D. changed
✅ 正确答案： D）changed
📝 解析： "the concentrations of these pollutants were altered" — altered = 改变。
💡 解题技巧： 词汇题：alter = change = modify。
问题 45： Natural pollutants can play an important role in controlling air pollution for which of the following reasons?
A. They function as part of a purification process.
B. They occur in greater quantities than other pollutants.
C. They are less harmful to living beings than are other pollutants.
D. They have existed since the Earth developed.
✅ 正确答案： A）They function as part of a purification process.
📝 解析： "These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil"。
💡 解题技巧： 因果细节题定位"serve as an air purification scheme"。
问题 46： According to the passage, which of the following is true about human-generated air pollution in localized regions?
A. It can be dwarfed by nature's output of pollutants in the localized region.
B. It can overwhelm the natural system that removes pollutants.
C. It will damage areas outside of the localized regions.
D. It will react harmfully with naturally occurring pollutants.
✅ 正确答案： B）It can overwhelm the natural system that removes pollutants.
📝 解析： "human output may be dominant and may temporarily overload the natural purification scheme"。
💡 解题技巧： 细节题定位"overload"。
问题 47： The word "noxious" in line 19 is closest in meaning to
A. harmful
B. noticeable
C. extensive
D. weak
✅ 正确答案： A）harmful
📝 解析： "increased concentration of noxious chemicals" — noxious = 有害的。
💡 解题技巧： 词汇题：noxious = harmful = toxic。
问题 48： According to the passage, the numerical value of the concentration level of a substance is only useful if
A. the other substances in the area are known
B. it is in a localized area
C. the naturally occurring level is also known
D. it can be calculated quickly
✅ 正确答案： C）the naturally occurring level is also known
📝 解析： "the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally"。
💡 解题技巧： 细节题定位"until we know"后的条件。
问题 49： The word "detectable" in line 24 is closest in meaning to
A. beneficial
B. special
C. measurable
D. separable
✅ 正确答案： C）measurable
📝 解析： "sulfur dioxide has detectable health effects at 0.08 ppm" — detectable = 可检测到的/可测量的。
💡 解题技巧： 词汇题：detectable = measurable = perceptible。
问题 50： Which of the following is best supported by the passage?
A. To effectively control pollution, local government should regularly review their air pollution laws.
B. One of the most important steps in preserving natural lands is to better enforce air pollution laws.
C. Scientists should be consulted in order to establish uniform limits for all air pollutants.
D. Human activities have been effective in reducing air pollution.
✅ 正确答案： A）To effectively control pollution, local government should regularly review their air pollution laws.
📝 解析： 首段说空气污染定义需灵活且持续变化，随着技术和对化学物质健康影响知识的增加，污染物清单不断加长，暗示法律需定期审查。
💡 解题技巧： 推理题结合首段关于定义变化的论述。
🌐 中文翻译
空气污染物的一个有用定义是：人类直接或间接添加到大气中的化合物，其数量足以对人类、动物、植被或材料造成不利影响。空气污染需要一个非常灵活的定义，允许持续变化。当英国在14世纪制定第一批空气污染法律时，空气污染物仅限于可以看到或闻到的化合物——与今天所知的有害物质广泛清单相去甚远。随着技术的发展和关于各种化学物质对健康影响的知识的增加，空气污染物的清单也在加长。将来，在某些条件下，甚至水蒸气也可能被认为是一种空气污染物。 许多更为重要的空气污染物，如硫氧化物、一氧化碳和氮氧化物，都存在于自然界中。随着地球的发展，这些污染物的浓度通过各种化学反应而改变；它们成为生物地球化学循环的组成部分。这些循环通过允许化合物从空气转移到水或土壤中，起到空气净化方案的作用。在全球范围内，自然界这些化合物的产出远远超过人类活动产生的数量。然而，人类生产通常发生在局部区域，如城市。在这个局部区域，人类产出可能占主导地位，并可能暂时使自然净化方案超负荷。结果是空气中有害化学物质的浓度增加。 出现不利影响的浓度将高于在没有人类活动的情况下污染物本应具有的浓度。某种物质要成为污染物，其实际浓度并不需要很大；事实上，数值本身告诉我们的信息很少，直到我们知道这代表了该地区自然发生的浓度增加了多少。例如，二氧化硫在百万分之0.08 (ppm) 时就有可检测到的健康影响，这大约是自然水平的400倍。然而，一氧化碳的自然水平为0.1 ppm，通常直到其水平达到约15 ppm时才成为污染物。
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