a monopolistically competitive firm is operating in the short run at the optimal level of output and is earning positive economic profits. which of the following describes how this firm will adjust in the long run? a entry of new firms shifts the firm's marginal cost and average cost curves downward, decreasing the firm's level of output, and decreasing the price the firm can charge until price equals average total cost. b entry of new firms shifts the firm's demand and marginal revenue curves leftward, decreasing the firm's level of output, and decreasing the price the firm can charge until price equals average variable cost. c entry of new firms shifts the firm's demand and marginal revenue curves leftward, decreasing the firm's level of output, and increasing the price the firm can charge until price equals average total cost. d exit of firms shifts the firm's demand and marginal revenue curves rightward, increasing the firm's level of output, and increasing the price the firm can charge until price equals average total cost. e entry of new firms shifts the firm's demand and marginal revenue curves leftward, decreasing the firm's level of output, and decreasing the price the firm can charge until price equals average total cost.