Rearrange the following four sentences (A), (B), (C) and (D) in the proper sequence to form a coherent paragraph.

(A) In emission trading, the government fixes the total amount of pollution that is acceptable to maintain a desired level of air quality.
(B) Economists argue this approach makes air pollution control more cost – effective than the current practice of fixing air pollution standards and expecting all companies to pollute below these standards.
(C) USA uses emission trading to control air pollution.
(D) It then distributes emission permits to all companies in the region, which add up to the overall acceptable level of emission.