How can carbon dioxide be considered a pollutant when it is a natural component of air?
a.
carbon dioxide is considered a pollutant even though it is a natural component of air, because it is harmful to all living organisms.
b.
carbon dioxide is considered a pollutant when larger concentrations than normal are present in the air as a result of human activity.
c.
carbon dioxide is considered a pollutant, because it forms harmful chemicals when mixed with other pollutants in the air.
d.
carbon dioxide cannot be considered a pollutant under any circumstances.