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.

对于流入道型（隔瓣下）室间隔缺损，超声最佳显示切面是（）。

A.左室长轴

B.心尖四腔心和五腔心

C.左室长轴与心底短轴

D.心底短轴

E.右室流出道长轴