The two leading producers of beer in the United States have market shares of 48 percent and 18 percent, respectively. On the basis of that information, we can conclude that:
A. the four-firm concentration ratio in the beer industry is nearly 100 percent.
B. the Herfindahl index in the beer industry exceeds 2,500.
C. firms practice overt collusion to set prices in the industry.
D. the industry is monopolistically competitive.