Answer: The engine size
Explanation:
Explanatory variable is a type of independent variable, but it may not always be independent. It is varied to determine its effect on the response variable. In this case, to establish a relationship between gas mileage of cars and the engine size, one will want to predict the average gas mileage for various engine size. Therefore, the engine size is the explanatory variable while the gas mileage is the response variable.
