Which of the following statements best describes environmental regulations that impose emissions limits on polluters?

a. They are a fair and efficient way to reduce pollution since all firms are treated equally.

b. They are likely to reduce pollution at a higher cost than would the imposition of per-unit pollution taxes.

c. They are more efficient than the use of pollution taxes since large polluters are less likely to respond to such taxes than are small polluters.

d. They are likely to reduce pollution at a lower cost than would the use of transferable pollution rights.