The marginal external cost per unit of effluent discharged into a river by a perfectly competitive chemical industry is currently estimated to be $50 per ton per year. Which of the following statements is true? Group of answer choices At the efficient output, the marginal social benefit of discharging effluent will be zero. Efficiency can be achieved with a $50 per ton annual effluent charge. Efficiency can be achieved by banning discharge of effluent. At the competitive equilibrium output, the marginal social benefit of discharging effluent is $50.