The market for good Q is perfectly competitive. However, it features negative externalities.

Consumers' marginal benefit is MB = 90-Q

Producers' marginal private cost is MC = Q.

The production of this good generates a marginal external cost MEC=4.

a) In the equilibrium of this market, the perfect competition quantity is Q_PC= _____

b) The socially efficient quantity is Q_SOC= _____

c) To achieve efficiency, the government can introduce a $ ____ per unit _____ (subsidy or tax)