Pollution reduction will be achieved for the least cost when

a. large polluters are required to reduce pollution by a greater extent than small polluters.

b. small polluters are required to reduce pollution by a greater extent than large polluters.

c. all firms are required to reduce pollution by a proportionate amount.

d. all firms are required to reduce pollution by an equal absolute amount.

e. the cost of reducing pollution by an additional unit is the same for all polluting firms.