The Dilemma of Natural Monopoly Occurs When

The dilemma of natural monopoly occurs when:

A) average costs rise as output expands.
B) a group of smaller producers has the potential to produce total industry output more efficiently than a single large firm.
C) demand equals supply at a point where the industry long-run average cost curve is still declining.
D) the profit-maximizing output level occurs at a point where the industry long-run average cost curve is still declining.