Q. Sometimes market activities (production, buying and selling) have unintended positive or negative effects outside market's scope. This is called an externality. Suppose that you are a policy maker concerned with correcting effects of gases and particulates emitted by and local power plant. What tools would you use? What would be benefits of action? What would be costs? How would you decide what was best level of emission reduction? Why do you think your approach would be better than others?