What is one reason that greenhouse gases in the Earth's atmosphere have increased? 

A)  reforestation in tropical areas 
B)  the restricted use of CFCs 
C)  planting of new seedlings 
D)  the burning of fossil fuels

Greenhouse Gases

Gases in Earth's atmosphere, such as carbon dioxide and methane, that trap heat, leading to a warming effect known as the greenhouse effect.

Fossil Fuels

Natural fuels such as coal, oil, and natural gas, formed from the decomposed remains of plants and animals over millions of years.

CFCs

Chlorofluorocarbons, chemical compounds once commonly used in refrigerants and aerosol sprays that contribute to ozone layer depletion.