[单选题]班应当指定（ ）。
A.A.安全员
B.B.观察哨
C.C.值日员
D.D.值班员

Text 4
Although, recent years have seen substantial reductions in noxious pollutants from individual motor vehicles, the number of such vehicles has been steadily increasing. Consequently, more than 100 cities in the U. S. still have levels of carbon monoxide, particulate matter, and ozone that exceed legally established limits. There is a growing realization that the only effective way to achieve further reduction in vehicle emissions—short of a massive shift away from the private automobile—is to replace conventional diesel fuel and gasoline with cleaner-burning fuel such as compressed natural gas, liquefied petroleum gas, ethanol, or methanol.
All of these alternatives are carbon-based fuels whose molecules are smaller and simpler than those of gasoline. These molecules burn more cleanly than gasoline, in part because they have fewer, if any, carbon-carbon bonds, and the hydrocarbons they do emit are less likely to generate ozone. The combust
A. Further attempts to reduce emission from gasoline-fueled vehicles will not help lower urban air-pollution level.
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.