For each of the following externalities, explain the underlying source of inefficiency (what effects are felt beyond the decision maker) and suggest a solution that could bring the activity to its efficient level:

a) Billboards along the highway detract from natural beauty.

b) Vaccinations received by domestic animals help wild animals avoid disease because they won't catch the disease from those vaccinated.

c) The use of e-mail communications as a substitute for letters reduces the number of trees harvested for paper production.