Categorize each policy response according to the type of regulation it best describes.
a. Auto manufacturers are allowed to pollute as much as they wish, provided that they have purchased a sufficient number of pollution licenses.
b. The government requires that auto manufacturers limit pollution to a specified threshold.
c. The government requires that auto manufacturers use a new, cleaner technology in producing cars.
d. An auto manufacturer is charged a fee per unit of pollution emitted into a river.