问答题 1.  As the world is getting more and more densely populated, air pollution has become a severe problem. Air pollution mainly derives from four major human activity sectors: industry, energy, transportation and agriculture. Energies are needed to run factories, to power trains, planes and buses. Nearly all of these energies are produced by burning fuels, which would cause air pollution. Scientists are studying new ways of generating electricity that can be less damaging to the environment. The increased public environmental awareness has led to the advent of activities such as recycling.