You Are Told That the Four-Firm Concentration Ratio in an Industry

You are told that the four-firm concentration ratio in an industry is 20. Based on this information you can conclude that

A) each of the top four firms has, on average, 20 percent of industry sales.
B) this industry's market structure is oligopoly.
C) the four largest firms account for 20 percent of industry sales.
D) each of the four largest firms accounts for 5 percent of industry sales.