The passage suggests which of the following about air pollution? A. Further attempts to reduce emissions from gasoline-fueled vehicles will not help lower urban air-pollution levels. B. Attempts to reduce the pollutants that an individual gasoline-fueled vehicle emits have been largely unsuccessful. C. Few serious attempts have been made to reduce the amount of pollutants emitted by gasoline-fueled vehicles. D. Pollutants emitted by gasoline-fueled vehicles are not the most critical source of urban air pollution. E. Reductions in pollutants emitted by individual vehicles have been offset by increases in pollution from sources other than gasoline-fueled vehicles.