Read the passage about polar bears.

Polar bears in the Arctic use the sea ice that covers the North Pole as a platform for hunting seals, their primary prey. Recent data suggest that Arctic sea ice may have thinned as much as 40% over the past 50 years. If polar ice continues to melt, it is likely that the polar bear will become extinct. Many scientists believe that the melting of the Arctic sea ice is due primarily to human activities.
Which of the following human activities is most likely to have the greatest effect on the thickness of the polar ice cap?
A. burning of fossil fuels
B. human population size
C. fishing in the Arctic Ocean
D. nuclear energy production