According to results of atmospheric testing originally published in 1973, which of the following best explains the major cause of gradual warming of Earth's atmosphere?
I. An increase in the amount of carbon dioxide in the lower atmosphere
II. An increase in the amount of nitrogen oxides in the upper atmosphere
III. An increase in sea surface temperatures in North America, leading to rising sea levels and storms
IV. Increased solar radiation
A. III only
B. I only
C. II and III only
D. None of the above