In longmn 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 0 a. barriers to entry and exit prevent firms from earning positive or negative economic profit 0 b. a positive profit would induce rms to produce less, decreasing price and profit, and a negative profit would induce firms to produce more, increasing price and profit 0 c. firms are price takers, maximizing profit by producing where price equals average cost 0 d. a positive profit would induce firms to enter, decreasing price and profit. and a negative profit would induce firms to exit, increasing price and profit 0 e. firms are price takers, maximizing profit by producing where price equals marginal cost