As a general rule, oligopoly exists when the four-firm concentration ratio

A) equals the Herfindahl index.
B) yields a Herfindahl index below 500.
C) is 40 percent or more.
D) is 50 percent or more.

Four-Firm Concentration Ratio

A metric that measures the total market share held by the top four firms within a specific industry, indicating the level of market concentration.

Herfindahl Index

A measure of market concentration used to determine the competitiveness of an industry, calculated by squaring the market share of each firm competing in the market.

Oligopoly

A market structure characterized by a small number of firms that dominate the market, leading to limited competition and potential for collusion.