You Are Planning an Experiment to Determine the Effect of the Brand

You are planning an experiment to determine the effect of the brand of gasoline and the weight of a car on gas mileage measured in miles per gallon. You will use a single test car, adding weights so that its total weight is 3000, 3500, or 4000 pounds. The car will drive on a test track at each weight using each of Exxon, Sunoco, and Texaco gasoline.
In this study, gasoline brand is a(n)

A) response variable.
B) explanatory variable.
C) lurking variable.
D) quantitative variable.