(a) Under the Clean Air Act, an agency established by two or more States, or by two or more municipalities located in different States, having substantial powers or duties pertaining to the prevention and control of air pollution.

(b) Under the Federal Water Pollution Control Act, an agency of two or more States established by or pursuant to an agreement or compact approved by the Congress or any other agency of two or more States, having substantial powers or duties pertaining to the control of pollution of waters.

(c) Under the Resource Conservation and Recovery Act, an agency of two or more municipalities in different States or an agency established by two or more States, with authority to provide for the disposal of solid wastes and serving two or more municipalities located in different States.

(d) In all other cases, an agency of two or more States having substantial powers or duties pertaining to the control of pollution.