Which of the following most accurately describes the competitive structure that is characterized by a firm that operates with the lowest average total cost and has the capacity to produce all of an industry's output（）

A. Oligopoly.

B. Competitive monopoly.

C. Natural monopoly.

下列哪项不属于过度考试焦虑的危害（）。

A.影响人际交往

B.易分散注意力

C.危害心理健康

D.危害身体健康