Monopolists usually earn economic rent

Using the chained-dollar method to calculate real GDP, real GDP is calculated by

A) valuing the current output at last year's real GDP prices. B) valuing the current output at current year prices. C) averaging the growth of output from one year to the next when the growth rates are calculated using this year's prices and using last year's prices. D) either A or C, depending which gives the larger value for real GDP. E) averaging the value of current output valued using base year prices and current output valued using current year prices.

Which theorem asserts that private economic actors can often solve the problem of externalities among themselves?