The Cheese Business in Lake Fon-Du-Lac, Wisconsin, Is a Competitive

The cheese business in Lake Fon-du-lac, Wisconsin, is a competitive industry.All cheese manufacturers have the cost function C = Q² + 9, while demand for cheese in the town is given by Q_d = 120 - P.The long-run equilibrium number of firms in this industry is

A) 120.
B) 38.
C) 19.
D) 34.
E) 39.