Which of the following is a greenhouse gas? 

A)  oxygen 
B)  hydrogen 
C)  methane 
D)  nitrogen

Which of the following is most similar in volume to an acre-foot of water?

A) 20 million gallons
B) 9 football fields covered with one-tenth of a foot of water
C) an acre covered with 3 feet 1 meter) of water
D) a square mile covered with 1 foot of water