2. 外语
  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 U. S. still have levels of carbon monoxide, particulate matter, and ozone that exceed legally established limits. There is a growing realization that the only effective way to achieve further reduction in vehicle emissions — short of a massive shift away from the private automobile — is to replace conventional diesel fuel and gasoline with cleaner-burning fuel 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 price 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 alone" 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 alone" vehicles fueled with methanol, they would need comparatively less fuel. Vehicles incorporating only the simplest of the engine improvements that makes methanol feasible would still contribute to an immediate lessening of urban air pollution.
According to the passage, incomplete combustion is more likely to occur with gasoline because ______.
选项 A、the combustion of gasoline releases photo chemically active hydrocarbons
B、the combustion of gasoline involves an intricate series of reactions
C、gasoline molecules have fewer carbon-carbon bonds
D、gasoline is a carbon-based fuel with smaller molecules
答案B
解析 事实细节题。本题的答案在第二段第三句和第四句：含有多条碳链的大分子的燃烧包含着一系列的复杂反应，这些反应增加了不充分燃烧的可能性。这句话与选项B 是一致的。
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