31. As a general rule, oligopoly exists when the four-firm concentration ratio: a. exceeds the Herfindahl index. b. is less than the Herfindahl index. c. is 40 percent or more. d. is 15 percent or more.

31. As a general rule, oligopoly exists when the four-firm concentration ratio:
a. exceeds the Herfindahl index.
b. is less than the Herfindahl index.
c. is 40 percent or more.
d. is 15 percent or more.

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