Suppose That Bill, George, And $\mathrm { Al }$ Constitute the Entire Market for Consumers of National Defense

Multiple Choice

Suppose that Bill, George, and $\mathrm { Al }$ constitute the entire market for consumers of national defense. Each man has an identical demand curve for national defense, which can be expressed as $P =$ $50 - Q$ . Suppose that the marginal cost for national defense can be expressed as $M C = \$ 30$ . What is the optimal quantity of national defense?

A) 150 units.
B) 60 units
C) 40 units.
D) 20 units.