The Smith family has three vehicles. Jeff fills up his vehicle for x dollars a gallon. Valerie also fills up her vehicle for x dollars a gallon. Megan fills up her vehicle for y dollars a gallon. The expression below represents the total cost of the Smith family filling up all of their vehicles with gasoline. ay + bx What is the best interpretation of bx in the above expression? A. The total amount it costs Megan to fill up her vehicle. B. The total amount it costs Jeff and Megan to fill up their vehicles. C. The total amount it costs Valerie to fill up her vehicle. D. The total amount it costs Jeff and Valerie to fill up their vehicles.