Suppose a ten firm industry has total sales of $50 million per year. The two largest firms each have sales of $10 million, the third largest firm has sales of $8 million, and the fourth largest firm has sales of $2 million. If the fifth through tenth largest firms combined have annual sales of $20 million, the four-firm concentration ratio for this industry is A. 25 percent. B. 45.7 percent. C. 60 percent. D. none of the above