If You Clean the Fuel Injectors of a Particular Car

Multiple Choice

If you clean the fuel injectors of a particular car, it increases the miles per gallon by two. Letting a driver skilled at increasing mpg take the car around the course increases mpg by four. Suppose the injectors were cleaned and a skilled driver drove the car. Which outcome below represents the one in which there is no interaction between the two variables?

A) increase six mpg
B) increase four mpg or two mpg
C) increase two mpg
D) decrease by two mpg.