The Difference Between Primary and Secondary Air Pollutants Is That

The difference between primary and secondary air pollutants is that primary air pollutants:

A) only affect plants, while secondary pollutants affect plants and animals.
B) are not harmful to humans, whereas secondary air pollutants may be toxic to humans.
C) enter the atmosphere directly, whereas secondary pollutants form from other substances released into the atmosphere.
D) are the direct result of natural processes, whereas secondary air pollutants are the result of human activity.
E) are smaller, primary molecules.