These reactions increase the probability of incomplete combustion and are more likely to（）uncombusted and photochemically active hydrocarbon compounds into the atmosphere.

There is a growing realization that the only effective ...

单项选择题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（）.