One measure of the extent of competition in an industry is the concentration ratio. What level of concentration indicates that an industry is an oligopoly? Most economists believe that a four-firm concentration ratio of oligopoly. (Enter your response as an integer.) than percent indicates that an industry is an Is the concentration ratio an accurate measure of the extent of competition? The four-firm concentration ratio OA. is flawed in that it includes sales in the U.S. by foreign firms. B. is flawed in that it includes competition between industries. C. is accurate because it is based on national and global competition. D. is accurate because it is based on estimates from the U.S. Census Bureau. OE. is flawed in that it is calculated for the nation market even though competition in some industries is local.