All of the following are sources of air pollution, except
a) chlorofluorocarbons (cfcs).
b) heavy metals such as mercury.
c) nitrogen oxides from automotive exhaust.
d) small particles of smoke and dust in smog.

The correct answer is B) Heavy metals such as mercury.

Explanation:

Heavy metals such as mercury or lead are denser than regular metals, this causes it is common for these metals to be found in the surface of Earth rather than in the air or atmosphere. Indeed, mercury and other similar metals are extracted from the Earth's crust and then used for different purposes. As a result of this, mercury ends up in the surface of Earth or in water sources which causes soil or water pollution. This means mercury is not a common source of air pollution; on the opposite other such as CFCs, nitrogen oxides and smog remain in the air and therefore are sources of air pollution.