The Clean Air Act of 1972 Required Some Companies to Install

The Clean Air Act of 1972 required some companies to install the "best available" pollution control technologies.This was an example of:

A) the government using a subsidy to encourage a negative externality.
B) the government using the command approach to discourage a negative externality.
C) the government using the command approach to encourage a positive externality.
D) a marketable pollution permit.
E) a pollution tax.