Driving under the influence is a more severe problem among college students at rural and suburban universities than at urban universities. Social scientists wanted to determine how much of the problem is due to the distance students must drive to obtain alcohol. They looked at police records for incidents involving students at a number of universities, and compared those with the mean distances between student housing and bars and liquor stores. The longer the distances, the more likely students were to be arrested or involved in accidents.

Which of the following, if true, would it be most important to take into account in evaluating the result?
(A) Many college students do not purchase alcohol at bars or liquor stores.
(B) Students at rural, suburban, and urban universities are arrested for driving under the influence more frequently than are non-student members of their communities.
(C) The average enrollment at urban universities is greater than that at rural and suburban universities.
(D) Local sheriff's departments near rural and suburban universities devote more resources to identifying and arresting those driving under the influence than do urban police forces.
(E) The number of bars and liquor stores per square mile is nearly five times as high in urban areas as it is in rural and suburban areas