Which of the following statements is TRUE? Select one: a. The presence of positive economic profit in a perfectly competitive market is consistent with the characteristics of a long-run competitive equilibrium b. When firms in a perfectly competitive market incur economic losses, some will exit in the long run, thereby shifting the industry supply curve rightward c. If a profit maximizing firm in a perfectly competitive market is incurring an economic loss, then it must be producing at a level of output where price is greater than average total cost. d. If a profit-maximizing firm in a perfectly competitive market is making an economic profit, then it must be producing at a level of output where price is greater than average total cost