Final answer:
In a natural monopoly, the marginal cost curve may not be positively sloped due to significant economies of scale. It could be flat or decreasing within the range of output that satisfies market demand, deviating from the typical upward slope observed in other market structures.
Step-by-step explanation:
In a natural monopoly, the marginal cost curve can behave differently than in other market structures. Typically, the marginal cost curve is upward sloping in many industries, reflecting increased costs as production expands. However, in a natural monopoly, average costs decline over the range that satisfies market demand due to economies of scale. In fact, a natural monopoly arises when the quantity demanded in the market is only large enough for a single firm to effectively and efficiently meet that demand at the minimal point of the long-run average cost curve. As such, the marginal cost can be much lower, sometimes even below the average cost, especially in a range where economies of scale are significant.
Therefore, while we might expect a positively sloped marginal cost curve in a typical industry, this is not always the case for a natural monopoly. In a natural monopoly, because of significant economies of scale and the extensive fixed costs involved, the marginal cost curve may actually be fairly flat or even decreasing over the relevant output range.