What Is the Major Advantage of a Cap-And-Trade System

What is the major advantage of a cap-and-trade system?

A) It reduces costs for companies under the cap.
B) It puts companies under the cap at a competitive advantage when foreign competitors are outside the cap.
C) It allows developed countries to meet greenhouse gas reproduction pledges by paying for carbon offset projects in developing nations.
D) It actually reduces overall emissions as the cap is lowered.