Automobiles, as a product of modern civilization, have been playing a vital part in the daily activities of human society. Industry needs automobiles for farming as well as transportation. Every day, automobiles drive people to and from work. On weekends automobiles take families for joyful outings. And during holiday seasons, automobiles fill the highways everywhere, even in remote areas. It could be said that the wheels of automobiles move society forward.

　　But automobiles have also given rise to a series of problems. For example, cities are overcrowded with automobiles. They create a lot of noise and traffic accidents which disable(伤残) or kill many people each year. Worst of all perhaps is the air pollution caused by the exhaust gases produced by automobiles. Some of the gases are highly toxic(有毒的) and a serious health hazard.