Which one of the following statements about air pollution in Canada is true:

Question $21$ options:

a$)$

Environmental racism doesn't exist in Canada because pollutant levels have decreased

b$)$

Canadians are breathing cleaner air, but some groups tend to have worse air quality than others

c$)$

$95\%$ of people are exposed to air that fails to meet the World Health Organization's standards

d$)$

Ammonia levels have decreased more than sulphur oxides