Earth Has Been Gradually Warming Over the Past Few Decades

Earth has been gradually warming over the past few decades. Based on a great deal of evidence, scientists believe that this warming is caused by .

A) the human release of chemicals called CFCs into the stratosphere
B) the fact that our politicians spout a lot of hot air
C) human activities that are increasing the concentration of greenhouse gases in Earth's atmosphere
D) the increase in forest fires during recent years