What was the largest single source of air pollution around the world by the 1990s?
a. meta smelting,
b. waste incineration,
c. auto emissions,
d. chemical emissions

The correct answer for the question that is being presented above is this one: "c. auto emissions." The largest single source of air pollution around the world by the 1990s is cause by auto emissions. There are many cars run around the streets everyday.