Qusetion: A classic way of introducing pollution regulation in to an urban area is the "rollback" method. First, you determine the maximum tolerable level of particular pollutant, xs. Next you find the maximum current concentration of that pollutant in the urban area xm. Presumably, xm > xs (otherwise, there is no problem). Regulations then require all polluters in the city to reduce emissions from their current level of em to es = em(xs/xm). Explain how this will bring pollution levels down to an acceptable point What might be the efficiency problems with such an approach?