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.

中包临界液面，不同铸机（）。

A.中包大小相同就相同

B.中包液面深度相同就相同

C.不受铸机影响

D.受拉速、液面深度、水口直径影响