K DOJ and FTC use the Hertindahl-Hirschman Index as a measure of concentration. How is this calculated? By O A. adding up the market shares of all firms in the industry, squaring this number and then dividing by the number of firms in the industry. OB. determining the market shares of the four largest firms in the industry, but unlike the concentration ratio, the Index includes sales in the United States by foreig OC. squaring the four - firm concentration ratio of the industry and dividing this number by the total number of firms in the industry. D. squaring the market shares of each firm in an industry and then adding up the values of the squares