The Herfindahl-Hirschman Index is a measure of market power that focuses on:

(A) the ratio of the price of a firm's product to the price elasticity of demand for the product.
(B) the share of the market controlled by the X largest firms in the market.
(C) the sum of the squares of the market share of each firm in an industry.
(D) the difference between a firm's product price and its marginal costs of production.