Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A), B), C) and D). You should decide on the best choice and mark the corresponding letter on Answer 2 with a single line through the centre.
Passage 1
A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or materials adversely. 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 substances 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 nitrogen oxides, are found in nature. As the Earth developed, the concentrations of these pollutants were 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 resulting from human activities. However, human production usually occurs in a localized area, such as a city.
In this localized 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 advert 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 substance to be a pollutant; in fact the numerical value tells us little 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 (25) 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.
21. 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.
22. 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 smelledC) the definition of air pollution will continue to change
D) a substance becomes an air pollutant only in cities
23. 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.
24. 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.
25. 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.
Passage 2
Foot racing is a popular activity in the United States. It is seen not only as a competitive sport but also as a way to exercise, to enjoy the companionship of like-minded people, and to donate money to a good cause. Tough serious runners may spend months training to compete, other runners and walkers might not train at all. Those not competing to win might run in an effort to beat their own time or simply to enjoy the fun and exercise. People of all ages, from those of less than one year (who may be pushed in strollers) to those in their eighties, enter into this sport. The races are held on city streets, on college campuses, through parks, and in suburban areas, and they are commonly 5 to 10 kilometers in length.
The largest foot race in the world is the 12-kilometer Bay to Breakers race that is held in San Francisco every spring. This race begins on the east side of the city near San Francisco Bay and ends on the west side at the Pacific Ocean. There may be 80 000 or more people running in this race through the streets and hills of San Francisco. In the front are the serious runners who compete to win and who might finish in as little as 34 minutes. Behind them are the thousands who take several hours to finish. In the back of the race are those who dress in costumes and come just for fun. One year there was a group of men who dressed like Elvis Presley, and another group consisted of firefighters who were tied together in a long line and who were carrying a fire hose. There was even a bridal party, in which the bride was dressed in a long white gown and groom wore a tuxedo. The bride and groom threw flowers to bystanders, and they were actually married at some point along the route.
26. The main purpose of this passage is to ..
A）encourage people to exercise
B）describe a popular activity
C）make fun of runners in costume
D）give reasons for the popularity of footraces
27. Which of the following in NOT implied by the author?
A）Footraces appeal to a variety of people.
B）Walkers can compete for prizes.
C）Entering a race is a way to give support to an organization.
D）Running is a good way to strengthen the heart.
28. Which of the following is NOT mentioned in this passage?
A）Some runners looked like Elvis Presley.
B）Some runners were ready to put out a fire.
C）Some runners were participating in a wedding.
D）Some runners were serious about winning.
29. A “bystander” in the last sentence refers to which of the following?
A）A walker. B）A participant.
C）A spectator. D）A judge.
30. Which of the following best describes the organization of this passage?
A）Chronological order. B）Specific to general.
C）Cause and result. D）Statement and example.
Passage 1
短文大意：文章首先介绍了空气污染物一词，而对于这一名词的定义从十五实际到现在是在不断的变化之中的，而且很多更重要的污染物如氧化硫、一氧化碳和氧化氮都是能在自然界中发现的。随着地球的进化和发展，污染物的浓度受到不同化学反应的影响而发生变化，并且成为生物地球化学循环的组成部分。而人类产生的一些东西，如垃圾、排泄物等是主要的污染物，会使自然净化过程暂时超载，最终的结果就是使空气中有毒的化学物质增长。
21. 答案为B)。主旨题。文章第一段指出空气污染物一词，第二段列举了一些排泄物，第三段提到了人类排放物，综合这三段的大意，可以得出本题的正确答案是B）。
22. 答案为C)。推断题。文章第一段主要讲了空气污染物的定义及定义的发展和变化，所以可以推断空气污染物的定义还是会不断变化的。故本题的正确答案是C）。
23. 答案为A)。文章第二段指出：“很多更重要的空气污染物如氧化硫、一氧化碳和氧化氮都能在自然界中发现。随着地球的进化和发展，污染物的浓度受到不同化学反应的影响而发生变化，并且成为生物地球化学循环的组成部分。生物地球化学循环是空气净化的一种方式，它使循环中涉及到的化合物从空气转移到水里或土里。在全球范围内，自然界产生的化合物远远多于人类活动产生的化合物。”由此可以推断自然污染物对于空气污染的控制起着重要的作用是因为这些物质是空气净化过程的一部分，所以本题的正确答案是A）。
24. 答案为B)。推断题。文章第三段第一句“In this localized region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles”（在地方性区域中，人类排放物可能占支配地位，可能暂时是自然循环中的净化负担过重），从这里不难看出本题的正确答案是B）。
25. 答案为A)。推断题。通读全文，根据文章的意思，不难推断出本题的正确答案是A）。
Passage 2短文大意：在美国，跑步是一种非常流行的运动。它不仅仅被视作一种竞赛运动，还被视作是一种途径：用来锻炼、享受与你有相同兴趣的人的陪伴与募捐。参加这项运动的人来自各个年龄层，也不需通过专门训练。竞赛的场地既可在城市的街道、大学的校园，也可在公园里或郊区。全长通常5到10公里。世界上规模最大的跑步比赛是每年春天在旧金山举办的12公里的公路赛。比赛队伍中最前面的是真正的竞赛者，中间的是一般参赛者，而队伍的最后是穿着各式各样服装的人们，如装扮成“猫王”、消防队员等，有一年，甚至有一对新人穿着礼服参赛，并举行了婚礼。
26．答案为B）。主旨题。这篇短文没有鼓励人们多锻炼的含义，也没有对赛跑这一运动项目为什么流行作充分的解释，因此A）D）项不正确；C）项文中提到“fun”这一词语，但没有嘲笑运动员之意。从全篇看，作者只是在客观描述赛跑这一深受人们欢迎的活动。
27．答案为D）。推断题。文中提到的内容有：A）参加赛跑的人员包括各个年龄层、各种各样的人；B）有些人参加赛跑是为了取胜，表明他们可以获得奖品；C）项可从“donate money to a good cause”得知文中已经提到。
28．答案为B）。推断题。文中提到了拿着消防水龙带（fire hose）的消防队员，但并没有说这水龙带与水龙头是连接在一起的，因而不能说这些参赛消防队员已经救火作好了准备。
29．答案为C）。词汇题。“bystander”和“spectator”都指观看而不是参加比赛的人。
30. 答案为D）。主旨题。文章第一段对赛跑作了大致的描述；而第二段以“the Bay to Breakers race”这一具体赛事来说明第一段的描述。
资料来源：新大学英语四、六级考点透视丛书：四级阅读/李华田主编，武汉大学出版社，2006
