20. [8] US coal fired electricity plants must buy tradable permits in order to have the right to emit sulfur oxides. What characteristic of the marginal damage from sulfur oxide emissions could make it more socially efficient to limit the emissions through a tradable permit system rather than by taxing the emissions? Explain why that characteristic makes it likely that a permit system is more socially efficient. Explain why sulfur oxide emissions might have that characteristic. You can illustrate your answer with a graph