The Introduction of Automatic Elevator Equipment Allowed Firms to Handle

The introduction of automatic elevator equipment allowed firms to handle the movement of people in a multistory building at less cost, thus decreasing the demand for elevator operators. The best explanation for this change is that the

A) marginal product of elevator operators was equal to its price.
B) marginal product of automatic elevator equipment was equal to its price.
C) marginal product of automatic elevator equipment divided by its price was greater than that for elevator operators.
D) marginal product of elevator operators divided by its price was greater than that for automatic elevator equipment.