Read the Scenario and Answer the Accompanying Questions

Multiple Choice

Read the scenario and answer the accompanying questions.
The trapping of heat in the lower atmosphere (the troposphere) , referred to as the greenhouse effect, is a natural process that helps regulate the temperature of our planet, allowing for life on Earth to exist and flourish. Nitrogen and oxygen molecules make up 99% of Earth's atmosphere, with water and carbon dioxide accounting for less than 0.5%. This natural greenhouse effect is the result of heat absorption by H2O and CO2 molecules in the lower atmosphere, and re-radiation of this trapped heat back to Earth's surface. Without the natural greenhouse effect, Earth's temperature would be 0°F instead of its present 57°F. The question for some is whether human activities are causing an increase of the greenhouse effect, and thus global climate change. Scientific data collected in the past 50 years indicates that the concentration of greenhouse gases in the atmosphere has been increasing
-Which greenhouse gas is most abundant naturally?

A) nitrous oxide
B) carbon dioxide
C) water vapor
D) trace gases
E) methane