Pollutants coming from automobile operation have begun to cause many environmental problems. It has been calculated, for example, that 70% of the carbon monoxide , 45% of the nitrogen oxides , and 34% of the hydrocarbon   pollution in the United States can be traced directly to automobile exhausts  . In addition, rubber, motor oil and other materials accumulate on roadways and are washed into streams, with effects nearly as serious as those of untreated waste water. In an effort to improve the situation, the U.S. government has made regulations on the use of the constituents   of automobile exhaust gas that are known to cause air pollution. These constituents fall roughly into three types: hydrocarbons that pass through the engine unburned; carbon monoxide, also a product of incomplete burning; and nitrogen oxides which are formed when nitrogen and oxygen are in contact at high temperatures. Besides their own poisonous character, hydrocarbons and nitrogen oxides react in the presence of sunlight to form harmful smog. Carbon monoxide and hydrocarbons are rather easily controlled by the use of higher burning temperatures in engines. Unfortunately, the conditions that produce minimum emission of hydrocarbons tend to raise emission of nitrogen oxides. In a way this difficulty is solved by adding recycled exhaust gas to the fuel mixture, thus avoiding the oversupply of oxygen that favors formation of nitrogen oxides. California, which has the most strict air-pollution laws in the United States, requires further special compounding of gas to control emissions, and several states have ordered that alcohol be mixed with gas, as this will reduce emissions of the carbon monoxide by 35 per cent and the hydrocarbons by 15 per cent. According to the text, alcohol is added to gas in order to   _  .
increase awareness of environment protection control air pollution increase the fuel efficiency replace gas gradually