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

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 amount of a per-unit subsidy would encourage the optimal societal consumption of the vaccine?

A) $\$ 1$ .
B) $\$ 2$ .
C) $\$ 4$ .
D) $\$ 6 .$