Automobile motor oil is either synthetic or conventional. An experiment is to be conducted to determine whether the mileage before an oil change is needed is the same for both types of oil. In previous studies , it was determined that car age (new, old) is associated with frequency of oil changes , but car size (small, medium , large) is not associated with frequency of oil changes. How would blocking best be used?

A. By blocking on car size
B. By blocking on car age
C. By blocking on mileage
D. By blocking on oil type
E. Blocking is not suggested