Suppose Al, Betty, and Carl Own the Only Fishing Companies

Suppose Al, Betty, and Carl own the only fishing companies in your village. Suppose the market price today is $10 per fish. Suppose Al catches 4,000 fish with an average total cost of $7.50, Betty catches 6,000 fish with an average total cost of $6, and Carl catches10,000 fish with an average total cost of $5. If everyone is a profit maximizer, what is Betty's marginal cost?

A) $6
B) $7
C) $8
D) $9
E) $10