If the Government Wanted to Efficiently Limit the Emission of Carbon

If the government wanted to efficiently limit the emission of Carbon Monoxide by all firms to exactly 4 Million tons it could

A) issue rights to pollute worth 4 Million tons and let the firms trade those rights in a market.
B) appeal to firms' environmental conscience to pollute less.
C) subsidize production.
D) rely on the Coase Theorem.