Which statement describes the greenhouse effect? 

A)  Heat trapped by greenhouse gases keeps the planet warm enough to support life. 
B)  The greenhouse effect is a new theory that explains the increase in UV radiation hitting the earth. 
C)  The greenhouse effect is increased by the increase in these two greenhouse gases, oxygen and carbon dioxide. 
D)  The greenhouse effect is increased by the amount of nitrogen gases in the troposphere.

The greenhouse effect refers to the process by which certain gases in Earth's atmosphere trap and re-radiate heat energy, keeping the planet warm enough to support life. Greenhouse gases include water vapor, carbon dioxide, methane, and others. This process has been occurring naturally since Earth's formation, but human activities (such as burning fossil fuels) have increased the concentration of greenhouse gases in the atmosphere, leading to a rise in global temperatures (known as global warming).