In long-run equilibrium, all firms in the industry earn zero economic profit. why is this true? all firms in perfectly competitive industries earn zero economic profit in the long run because a. firms are price takers, maximizing profit by producing where total revenue equals total cost. b. if profit were positive, then firms would produce more, increasing price, and if profit were negative, then firms would produce less, decreasing price. c. firms are price takers, maximizing profit by producing where price equals marginal cost. d. if profit were positive, then firms would enter, decreasing price, and if profit were negative, then firms would exit, increasing price. e. barriers to entry and exit prevent firms from earning positive or negative economic profit.