Short Passage: Please explain

A plant-based automobile fuel has just become available in Ternland. A car can be driven as far on a gallon of the new plant-based fuel as a car can be driven on a gallon of gasoline, but a gallon of the plant-based fuel both costs less and results in less pollution. Therefore, drivers in Ternland who switch to it will reduce the amount they spend on fuel in a year while causing less environmental damage.

Which of the following is an assumption on which the argument relies?

A- There is no expense associated with operating an automobile that is higher when the automobile uses the plant-based fuel than when it uses gasoline.

B- Automobiles that have been operated using the plant-based fuel can no longer be operated using regular gasoline.

C- The environmental damage attributable to automobiles is due almost entirely to the production and combustion of fuel automobiles use.

D- The advantages of the plant-based fuel over gasoline will not lead those who switch to the plant-based fuel to do more driving.

E- Most drivers in Ternland will switch from gasoline to the plant-based fuel.

I am wondering if someone can explain why answer choice D is correct here. Why can’t I choose A?

This is the conclusion:

drivers in Ternland who switch to it will reduce the amount they spend on fuel in a year while causing less environmental damage

They never argue that they will spend less on car costs in general, only fuel, so A isn’t necessary to make the argument.