边际成本是指产量（业务量）向（）变化时成本的变动数额。

A.无限小

B.无限大

C.0

D.无法确定

Before using a fixed CO₂ system to fight an engine room fire，you must（）.

A.secure the engine room ventilation

B.secure the machinery in the engine room

C.evacuate all engine room personnel

D.All of the above