If Government Regulators Guarantee That a Natural Monopoly Will Earn

If government regulators guarantee that a natural monopoly will earn normal profits, then

A) the monopolist has little or no incentive to hold down its costs.
B) the monopolist has a sharp incentive to hold down its costs so as to increase profits.
C) the monopolist will probably try its best to maintain its costs at the current level.
D) it is impossible to determine the monopolist's incentives regarding costs before the firm actually incurs some changes in its costs.