Although recent years have seen substantial reductions in noxious pollutants from individual motor vehicles, the number of such vehicles has been steadily increasing. Consequently, more than 100 cities in the United States still have levels of carbon monoxide, particulate matter, and ozone (generated by photochemical reactions with hydrocarbons from vehicle exhaust) that exceed legally established limits. There is a growing realization that the only effective way to achieve further reductions in vehicle emissions — short of a massive shift away from the private automobile — is to replace conventional diesel fuel and gasoline with cleaner-burning fuels such as compressed natural gas, liquefied petroleum gas, ethanol, or methanol.

All of these alternatives are carbon-based fuels whose molecules are smaller and simpler than those of gasoline. These molecules burn more cleanly than gasoline, in part because they have fewer, if any, carbon-carbon bonds and the hydrocarbons they do emit are less likely to generate ozone. The combustion of larger molecules, which have multiple carbon-carbon bonds involves a more complex series of reactions. These reactions increase the probability of incomplete combustion and are more likely to release uncombusted and photochemically active hydrocarbon compounds into the atmosphere. On the other hand, alternative fuels do have drawbacks. Compressed natural gas would require that vehicles have set of heavy fuel tanks — a serious liability in terms of performance and fuel efficiency — and liquefied petroleum gas faces fundamental limits on supply.

Ethanol and methanol, on the other hand, have important advantages over other carbon-based alternative fuels: they have higher energy content per volume and would require minimal changes in the existing network for distributing motor fuel. Ethanol is commonly used as a gasoline supplement, but it is currently about twice as expensive as methanol, the low cost of which is one of its attractive features. Methanol’s most attractive feature, however, is that it can reduce by about 90 percent the vehicle emissions that form ozone, the most serious urban air pollutant.

Like any alternative fuel, methanol has its critics. Yet much of the criticism is based on the use of “gasoline clone” vehicles that do not incorporate even the simplest design improvements that are made possible with the use of methanol. It is true, for example, that a given volume of methanol provides only about one-half of the energy that gasoline and diesel fuel do; other things being equal, the fuel tank would have to be somewhat larger and heavier. However, since methanol-fueled vehicles could be designed to be much more efficient than “gasoline clone” vehicles fueled with methanol they would need comparatively less fuel. Vehicles incorporating only the simplest of the engine improvements that methanol makes feasible would still contribute to an immediate lessening of urban air pollution.

1. The author of the text is primarily concerned with

[A] countering a flawed argument that dismisses a possible solution to a problem.

[B] reconciling contradictory points of view about the nature of a problem.

[C] identifying the strengths of possible solutions to a problem.

[D] discussing a problem and arguing in favor of one solution to it.

2. According to the text, incomplete combustion is more likely to occur with gasoline than with an alternative fuel because

[A] the combustion of gasoline releases photochemically active hydrocarbons.

[B] the combustion of gasoline embraces an intricate set of reactions.

[C] gasoline molecules have a simple molecular structure.

[D] gasoline is composed of small molecules.

3. The text suggests which of the following about air pollution?

[A] Further attempts to reduce emissions from gasoline-fueled vehicles will not help lower urban air-pollution levels.

[B] Attempts to reduce the pollutants that an individual gasoline-fueled vehicle emits have been largely unsuccessful.

[C] Few serious attempts have been made to reduce the amount of pollutants emitted by gasoline-fueled vehicles.

[D] Pollutants emitted by gasoline-fueled vehicles are not the most critical source of urban air pollution.

4. Which of the following most closely parallels the situation described in the first sentence of the text?

[A] Although a town reduces its public services in order to avoid a tax increase, the town’s tax rate exceeds that of other towns in the surrounding area.

[B] Although a state passes strict laws to limit the type of toxic material that can be disposed of in public landfills, illegal dumping continues to increase.

[C] Although a town’s citizens reduce their individual use of water, the town’s water supplies continue to dwindle because of a steady increase in the total populating of the town.

[D] Although a country attempts to increase the sale of domestic goods by adding a tax to the price of imported goods, the sale of imported goods within the country continues to increase.

5. It can be inferred that the author of the text most likely regards the criticism of methanol as

[A] flawed because of the assumptions on which it is based.

[B] inapplicable because of an inconsistency in the critics’ arguments.

[C] misguided because of its exclusively technological focus.

[D] inaccurate because it ignores consumers’ concerns.
[答案与考点解析]

1. 【答案】C

【考点解析】这是一道中心主旨题。本文的中心主旨句是首段的尾句。通过对首段尾句以及其它段落主题句的阅读和理解，可以推导出本题的正确选项是C。考生在破解此类题型时一定要首先抓住原文的中心主旨句以及各段的主题句。

2. 【答案】B

【考点解析】本题是一道指代词题型。根据题干中的“incomplete combustion”可将本题的答案信息来源迅速确定在第二段的第四句，该句中的指代词“these”暗示本题的具体答案信息来源应该在第二段的第三句。通过阅读和理解第二段的第三句可推导出本题的正确选项是B。考生在解题时对指代词一定要认真理解。

3. 【答案】A

【考点解析】本题是一道审题定位题型。通过题干中的“air pollution”可将本题的答案信息来源迅速确定在第一段，因为第一段首句中含有“noxious pollutants”，都在谈污染的问题。通过阅读和理解第一段，尤其是第一段尾句即全文的中心主旨句，可推导出本题的正确选项是A。这是一道比较难的题。考生在解题时一定要具备审题定位能力，还要具备归纳和推导能力。

4. 【答案】C

【考点解析】这是一道类比题型，因为本题题干中含有“parallels”(与……相当，与……一样)一词。本题题干已将类比的参照对象确定在第一段的首句。通过仔细阅读和理解第一段的首句，可推导出本题的正确选项是C，因为该选项涉及的增减关系的前提条件和原文一脉相承。考生在破解这种题型时应对原文所表达的内容与关系有明确的认识和把握，不可以无根据地推导。

5. 【答案】A

【考点解析】这是一道细节推导题型。本题题干中的“the criticism of methanol”把本题的答案信息来源确定在第四段的第一、二句。通过阅读和理解这两句话，尤其是第二句话，就可推导出本题的正确选项A。考生在破解本题时一定要注意第二句话的第一个词“Yet”在语意上所起的作用。考生在解题时一定要重视启承转合词的出现和功能。

[参考译文]

近年来，尽管单个机动车辆排放的有毒污染物已大幅减少，但这些机动车辆的数量一直在持续增长，结果是，在美国有100多个城市的大气中一氧化碳、灰尘和臭氧(由车辆尾气中的碳氢化合物经光化反应产生)的含量已超出了法定的限度。人们愈来愈认识到，实现车辆废气进一步减少而又不必大规模减少私家轿车的唯一有效途径是用清洁燃料来代替传统的柴油和汽油燃料。这些清洁燃料包括压缩天然气、液化石油气、乙醇和甲醇。

所有这些替代产品都是碳基燃料，不过它们的分子要比汽油的分子小且简单。它们可以比汽油燃烧得更彻底，部分原因在于，即使它们含有碳—碳化学键，含量也较少，而且它们释放的碳氢化合物也不大可能产生臭氧。那些具有多重碳——碳化学键的大分子在燃烧时包含了一系列复杂的反应。这些反应增大了不完全燃烧的可能，并且更容易释放未燃烧的碳氢化合物，而这些化合物又对光化学反应很活跃。另一方面，这些替代燃料也有自己的缺点。压缩天然气可能需要车辆配备一个笨重的油箱，这对车辆性能和燃料效率又极为不利，并且液化石油气还面临着供应上的根本限制。

从另一方面来看，甲醇和乙醇与其它的碳基替代燃料相比有重要的益处：它们具有单位体积更高的能量含量，而且对现存的燃料销售网络只需很小地改变。乙醇通常作为汽油的替代品，但目前它比甲醇要贵两倍，因此甲醇的低廉价格便成为引人注目的特点。然而，甲醇最吸引人的特点还在于它可以减少90%形成臭氧的废气，而臭氧又是城市中最严重的污染物。

和其它的替代燃料一样，甲醇也遭到了非难。可大多数批评都建立在使用“汽油系列”的车辆不进行最简单的设计改进以可能使用甲醇燃料的基础上。例如，事实上一定体积的甲醇和同体积的汽油和柴油相比，只能提供相当于它们大约一半的能量。在其它因素相同的条件下，油箱可能要大一点和重一点，然而，由于甲醇燃料的车辆可以设计得比使用甲醇的“汽油系列”车辆还要高效，它们可能需要更少的燃料。同时，对发动机进行最简单的改进便可使用甲醇的车辆，这将对城市大气污染的迅速减轻做出贡献。