A number of coal burning steam locomotives pass by several largely populated towns each day spewing sulfur dioxide. Each coal burning steam locomotive is owned by a different company.

Assume that the true marginal abatement cost curve for all firms is not known to a regulator. If the marginal damage from sulfur dioxide is fairly constant at all levels of sulfur dioxide levels, what is the better type of policy to implement, a quota or a tax on sulfur dioxide emissions? Give a brief explanation of why you have chosen that particular policy. Use a graph to support your explanation.