What is the Herfindahl-Hirschman index? It is

a. the list of oligopolistic industries according to their concentration ratios, from highest to lowest.

b. the list of mergers that occur in any year in oligopolistic industries, according to type, such as horizontal, vertical, or conglomerate.

c. a measure of market power in an industry, summing the squares of the market shares of all the firms in the industry.

d. a measure of market power in an industry, extending the concentration ratio from four leading firms to eight leading firms.

e. a list of oligopolistic industries identified by geographic location, such as Southeast, Midwest, or New England.