There is a growing realization that the only effective way to achieve further reductions in vehicle（）is to replace conventional diesel fuel and gasoline with cleaner-burning fuels.

Consequently, more than 100 cities in the United States...

单项选择题Consequently, more than 100 cities in the United States still have levels of carbon monoxide, particulate matter, and ozone that（）legally established limits.

Although recent years have seen substantial reductions ...

单项选择题Although recent years have seen substantial reductions in noxious pollutants from individual motor vehicles, the number of such vehicles（）.

Vehicles（）only the simplest of the engine improvements ...

单项选择题Vehicles（）only the simplest of the engine improvements that methanol makes feasible would still contribute to an immediate lessening of urban air pollution.