Gene's Car Wash Is a Natural Monopoly

Gene's Car Wash is a natural monopoly. To wash 100 cars a week, if Gene is unregulated, he would charge a price of $10. Gene's long-run average cost for washing 100 cars is $8, his average variable cost is $6, and his marginal cost is constant at $4. If Gene was regulated using a marginal cost pricing rule, the price he would be allowed to charge to wash 100 cars is

A) $10.
B) $8.
C) $6.
D) $4.