Pollution yields both costs and benefits. Here the curve MSC shows how the marginal cost to society as a whole from emitting one more ton of pollution emissions depends on the quantity of emissions. The curve MSB shows how the marginal benefit to society as a whole of emitting an additional ton of pollution emissions depends on the quantity of pollution emissions. The socially optimal quantity of pollution is Q_OPT; at that quantity, the marginal social benefit of pollution is equal to the marginal social cost, corresponding to $200.