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.

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.

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