Refer to the Diagrams Below in Which Figures (A) and (B)

Multiple Choice

Refer to the diagrams below in which figures (a) and (b) show demand curves reflecting the prices Alvin and Elmer are willing to pay for a public good, rather than do without it.If the marginal cost of the optimal quantity of this public good is $10, the optimal quantity must be:

A) 1 unit.
B) 2 units.
C) 3 units.
D) 4 units.