Assume there are two regions with the same marginal social cost curve for abatement of pollution by mobile sources. Assume that the marginal social benefit of abatement is higher in the "dirty" region than in the "clean" region. Compare the efficient leve

The efficient level of abatement in the two regions will not be the same. The efficient level of abatement in the "dirty" region will be higher because it has a higher marginal social benefit of abatement. This means that the social benefit of abatement is greater in the "dirty" region and therefore the amount of pollution reduced is also greater. In contrast, the efficient level of abatement in the "clean" region will be lower, since the marginal social benefit of abatement is lower. This implies that the amount of pollution reduced will also be lower in this region.