单选题 Which of the following is least relevant when explaining why monopoly firms can earn positive economic profits over the long term?

A. The ability to use price discrimination.
B. The existence of economies of scale.
C. Control over production input resources.

【正确答案】 A

【答案解析】High entry barriers due to economies of scale, government licensing, resource controls, and patents prevent new firms from entering the market to exploit positive economic profit opportunities.