Why Was the Problem of Acid Rain More Easily Resolved

Why was the problem of acid rain more easily resolved than other issues?

A) The pollution affected relatively few states,making cooperation easier.
B) The pollution affected a large number of states,which led to widespread concern.
C) It was inexpensive to reduce the emission of the pollutants that caused acid rain.
D) The major polluters were developing countries that were easily pressured by more powerful states to reduce pollution levels.
E) Firms were already moving away from using coal-burning plants and toward utilizing cleaner technology.