[单项选择]

Text 4
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 combust
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