Why Does the Book Say There Are "Two" Ozone Problems

Multiple Choice

Why does the book say there are "two" ozone problems?

A) Because there are two sources of the gas implicated in thinning the ozone layer.
B) Because there are two kinds of gases implicated in thinning the ozone layer.
C) Because there is a thinning of the upper atmosphere ozone layer and increasing ozone at ground level that produces photo-chemical smog.
D) Because ozone holes occur in two places above the earth, over the north and south poles.