(4) AP = MP Explanation: There is a close relationship between marginal product and average product because both are derived from total product. When marginal product is equal to average product, the average product is at its maxi-mum. In the short-run production function, since marginal product starts off as greater than average product and then falls below average product, we can assume that at the "cross-over point," when MP = AP, AP is at its maximum.