Branson: Most of the air pollution in this country comes from our largest cities  These cities would pollute less if they were less populated
So if many people in these cities were to move to rural areas, air pollution in the country as a whole would be reduced
Similarly, we could conclude that most of this city's air pollution would be eliminated if this city built a public transportation system. After all, public transportation produces much less pollution per passenger, and all automobile trips could be replaced by trips on public transportation.