Suppose That the Marginal Private Benefit of a New Vaccine $M P B = 10 - Q$

Multiple Choice

Suppose that the marginal private benefit of a new vaccine could be measured as $M P B = 10 - Q$ . Suppose that the marginal social benefit could be measured as $M S B = 12 - Q$ . Suppose that the private marginal cost curve could be measured as $M P C = Q$ . What quantity represents the optimal societal consumption of the vaccine?

A) 4 units.
B) 5 units.
C) 6 units.
D) 10 units.