Use this fact to answer the question: as mileage per galon of gasoline increases , the amount of gasoline used will decrease

Which of the following is true?

Pick from: (I) switching from a truck to a moped will save gas. (II) switching from a moped to a truck will save gas. (III) switching from a car to an SUV will save gas. (IV) switching from a car to a truck will save gas.

The answer is:
(I)