单项选择题
When stand-by engine instructions have been received, it’s necessary to make it sure that all valves in the various systems are（）
A.operational B.open C.closed securely D.at middle position
单项选择题 The monitoring function may serve all of the following purposes except（）
单项选择题 A feedback is an input（）
单项选择题 The closed-loop system differs from the open-loop system by（）
