An auto-parts store offers a fuel additive that claims to increase a vehicle’s gas mileage. The additive is poured into a vehicle’s gasoline tank after the tank is filled. To measure the claim, 30 taxi drivers receive a bottle of the additive to use the next time they fill their gas tank. The drivers then compare their gas mileage with the additive to their gas mileage before the additive was used. What is the response variable in this scenario?

the age of the car
the 30 cars used in the study
the use of the additive
the gas mileage after the additive is used