Suppose That Bill, George, and Al Constitute the Entire Market $P = 50 - Q$

Multiple Choice

Suppose that Bill, George, and 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.