GRE阅读题目解析:机动车尾气排放导致空气污染

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　　GRE阅读题目解析:机动车尾气排放导致空气污染

　　P48

　　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 a 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 a 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. According to the passage, 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 involves an intricate series of reactions

　　C. gasoline molecules have a simple molecular structure

　　D. gasoline is composed of small molecules.

　　E. gasoline is a carbon-based fuel

　　2. Which of the following most closely parallels the situation described in the first sentence of the passage?

　　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 population 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.

　　E. Although a country reduces the speed limit on its national highways, the number of fatalities caused by automobile accidents continues to increase.

　　3. It can be inferred from the passage that a vehicle specifically designed to use methanol for fuel would

　　A. be somewhat lighter in total body weight than a conventional vehicle fueled with gasoline

　　B. be more expensive to operate than a conventional vehicle fueled with gasoline

　　C. have a larger and more powerful engine than a conventional vehicle fueled with gasoline

　　D. have a larger and heavier fuel tank than a “gasoline clone” vehicle fueled with methanol

　　E. average more miles per gallon than a “gasoline clone” vehicle fueled with methanol

　　4. The passage 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.

　　E. Reductions in pollutants emitted by individual vehicles have been offset by increases in pollution from sources other than gasoline-fueled vehicles.

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