• a. sales; four

The concentration ratio is the percentage of sales earned by the four largest firms in the industry.

The concentration ratio measures the market share of the top four firms in an industry. Concentration ratios range from 0% to 100%. The concentration ratio is calculated by dividing the sales of the largest four firms and dividing by the number four. The four-firm concentration ratio is mostly used but also the alternative approach but also one can apply an eight-firm concentration ratio.