Answer the Question on the Basis of the Following Information

Answer the question on the basis of the following information for a public good. Pa and Pb are the prices that individuals A and B are willing to pay for the last unit of a public good, rather than do without it. These people are the only two members of society. If this good were a private good instead of a public one, the total quantity demanded at a $3 market price would be

A) 2 units.
B) 3 units.
C) 6 units.
D) 4 units.