A car can travel 500.0 miles on a tank of gasoline. When full, the car's gas tank holds 18.0 gallons of gasoline. What is the gas mileage for this car?

a) 500.0/18.0 miles per gallon
b) 18.0/500.0 gallons per mile
c) 500.0/18.0 gallons per mile
d) 18.0/500.0 miles per gallon