Suppose that in an industry, firm X has 50 percent market share, firm Y has 35 percent market share, and firm Z has 10 percent market share. Which of the following mergers is NOT likely to be challenged by the Federal Trade Commission?

a. a merger between firms X and Y

b. a merger between firms Y and Z

c. a merger between firms X and Z

d. Any merger of two firms among those firms is likely to be challenged.