Why is it harder to push a van filled with bowling balls than it is to push van filled with basketballs?

The mass of the bowling balls is greater than the mass of the basketballs.
Its more difficult to move objects of more mass. Since the vans are the same mass, the difference in mass has to be directly relative to the balls. Bowling balls have a denser core than basketballs which are filled with air.

The mass of the acr is greatest tahn tha mss of the vbasket

The answer to this question is one of many answers that could possibly lead to the conclusion of this question; with my keen knowledge of this topic i have concluded that the answer is not A,B,C, nor could it be D; it determines where this "van" of "balls" is; could it be in the vast space of a vaccume called space, or could it be on earth; thus far i have concluded the answer is Everything. It's b.