The first organisms that performed photosynthesis evolved about 2.2 billion years ago. During the next few million years, their actions caused which of these events?

A. an increase in oxygen levels, which led to the extinction of many anaerobic organisms
B. a decrease in oxygen levels, which led to the arrival of the first anaerobic organisms
C. an increase in carbon dioxide levels, which led to a warmer climate
D. a decrease in carbon dioxide levels, which led to the first ice age