In the natural gas industry, low average total costs are obtained only through large-scale production. In other words, the initial cost of setting up all the necessary pipes and hoses makes it risky and, most likely, unprofitable for competitors to enter the market.

A monopolist, unlike a competitive firm, has some market power. It can raise its price, within limits, without the quantity demanded falling to zero. The main source of a monopolist's market power is barriers to entry - that is, obstacles that make it difficult for competitors to enter the market. Which of the following best describes the source of the monopolist's market power in the preceding scenario?

a. Economies of scale
b. Innovation
c. Hard to duplicate resources