Option E is 200 million

C $4.00- A Demand Dollars per trip 2.50 h Long-run 1.50 g 1.25 average cost B 0.50-f e Long-run `Marginal revenue marginal cost 100 200 210 Trips per month (millions) Refer to Exhibit 15.1, which shows the cost and revenue curves for a natural monopolist-the operator system. At the efficient output rate, the efficient quantity is a) 210 million. b) 250 million. O c) 100 million. O d) 150 million