Choice D is correct. The circumference of the larger wheel is 18π inches (C = πd). After 72 revolutions, the larger wheel will have gone a distance of 72(18π) inches. The circumference of the smaller wheel is 12π inches, and if we call the number of revolutions that the smaller wheel makes r, then we know that 12πr = 72(18π). Dividing both sides by 12π gives us r = 6(18) or 108 revolutions. Note that in this problem we have used the relation distance = rate × time, where the time for both wheels is a fixed quantity.