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（）.

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.