Many observers believe that the levels of pollution in our society are too high.

Many observers believe that the levels of pollution in our society are too high. If society wishes to reduce overall pollution by a certain amount, which of the following reasons supports why it's efficient to have different amounts of reduction at different firms? a) The costs of reducing pollution differ across firms.
b) All firms should be allowed to produce some pollution.
c) Firms with the highest profit should bear the largest burden of reducing pollution.
d) The costs of reducing pollution are the same across firms.