单项选择题

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Cars account for half the oil consumed in the U.S., about half the urban pollution and one fourth the greenhouse gases. They take a similar toll of resources in other industrial nations and in the cities of the developing world. As vehicle use continues to increase in the coming decade, the U.S. and other countries will have to deal with these issues or else face unacceptable economic, health-related and political costs. It is un likely that oil prices will remain at their current low level or that other nations will accept a large and growing U.S. contribution to global climatic change.
Policymakers and industry have four options: reduce vehicle use, increase the efficiency and reduce the emissions of conventional gasoline powered vehicles, switch to less harmful fuels, or find less polluting driving systems. The last of these-in particular the introduction of vehicles powered by electricity-is ultimately the only sustainable option. The other alternatives are attractive in theory but in practice are either impractical or offer only marginal improvements. For example, reduced vehicle use could solve traffic problems and a host of social and environmental problems, but evidence from around the world suggests that it is very difficult to make people give up their cars to any significant extent. In the U .S., mass-transit ridership and carpooling have declined since World War Il . Even in western Europe, with fuel prices averaging more than $ 1 a liter (about $ 4 a gallon) and with easily accessible mass transit and dense populations, cars still account for 80 percent of all passenger travel.
Improved energy efficiency is also appealing, but automotive fuel economy has barely made any progress in 10 years. Alternative fuels such as natural gas, burned in internal-combustion engines, could be introduced at relatively low cost, but they would lead to only marginal reductions in pollution and greenhouse emissions (especially because oil com panics are already spending billions of dollars every year to develop less polluting types of gasoline) .
Which of the following is practical but only makes a marginal contribution to solving the problem of greenhouse emissions