Usually the panels are interlocked with （） so that the devices can be made dead before the doors are opened.

A.a closed loop control system
B.an open loop control system
C.a boiler control system
D.a steering gear control system

A.a manual closed loop
B.a manual opened loop
C.an automatic closed loop
D.an automatic opened loop

A.the absolute pressure
B.the gauge pressure
C.the vacuum pressure
D.the differential pressure