air pollution
　　air pollution is one of the major problems of the modern world. a great deal of energy is needed to run the factories of modern industrial nations. automobile, trains, planes and busses need energy, too. nearly all of this energy is produced by burning fuels. the burning produces wastes, some of which remain in the air, causing air pollution.来源：恒星英语学习网
　　scientists are finding ways to stop air pollution especially in big industrial areas and densely-populated cities. they are studying new ways of generating electricity that may be less damaging to the environment. in the meantime, many power plants are being modernized to give off less polluted material. also, engineers try to design and locate new power plants to do minimum damage to the environment.
　　however, it may be that we still have a long way to go before we have a "clean" world. nearly all of the countries are trying hard to prevent and control air pollution. there indeed seems to be no end to tasks that environmentalists will be required to do. people will continually find new ways to control pollution.