Assume there are 100 million passenger cars in the United States and the average fuel efficiency is 20 mi/gal of gasoline. If the average distance traveled by each car is 10000 mi/yr, how much gasoline would be saved per year if the average fuel efficiency could be increased to 25 mi/gal?
a) 2 billion gallons
b) 2.5 billion gallons
c) 5 billion gallons
d) 10 billion gallons