Which of the Following Statements About Natural Monopolies Is True

Which of the following statements about natural monopolies is true?

A) Natural monopolies occur because of strong economies of scale in production and distribution.
B) Over a large range of output, a natural monopolist's average, or unit, costs decrease as the level of output increases.
C) A natural monopolist can produce at a lower average cost than could any of several smaller sellers if they were competing in the market.
D) All of the above.