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 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 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-car-bon bonds and the hydrocarbons t
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
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