Which of the Following Statements Is Not Correct

Which of the following statements is not correct?

A) The real cost of producing X is the quantity of products, R, S, or T etc., which could have been produced with the resources devoted to X.
B) Diseconomies of scale arise primarily from the difficulties in managing and coordinating a large-scale business enterprise.
C) The law of diminishing returns explains the fact that the long-run average total cost curve is U-shaped.
D) Average fixed costs diminish so long as output increases.