A Paper Mill Discharges Chemicals into a River Which Pollutes

A paper mill discharges chemicals into a river which pollutes the shores of a downstream resort area.The private marginal cost,social marginal cost,and marginal benefit associated with the production of paper are given by the following equations: MC_P = 5 + Q
MC_S = 10 + 2Q
MB = 35 - 0.5Q.
The allocatively efficient level of output is ________ units.

A) 0
B) 10
C) 15
D) 25
E) 30