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  3. Although recent years have seen substantial reductions in noxious pollutants from individual motor vehicles, the number of such

Although recent years have seen substantial reductions in noxious pollutants from individual motor vehicles, the number of such

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问题     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 mom 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 reaction. These reactions increase the probability of incomplete combustion and are more likely to release un-combusted and photo-chemically 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 in 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 k 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, methanal has its critics. Yet much of the criticism is based on the use of "gasoline done" 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.
选项 A、countering a flawed argument that dismisses a possible solution to 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
答案D
解析 主旨题。题意为："本文作者主要关注______"。作者先是提到汽车尾气造成的污染，并提出一些解决办法。接着指出用甲醇代替汽油就能即刻大大减少城市的大气污染。可见选项"讨论一个问题并论述支持其中的一个解决方法"，为正确答案。
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