Smoke from wood fires is a major source of air pollution in southern Canberra. Suppose the ACT government is considering banning the use of wood fires because of the pollution they create. In making this decision, the government cares only about efficiency.

a Use a graph to illustrate the situation in which wood fires should be banned entirely.
b Use a graph to illustrate the situation in which the use of wood fires should be reduced from the free-market level but not banned.