The （） is a crystal which, under pressure, produces an electric current which varies with the pressure.

A.the system is manned manually
B.the system is situated remotely from the operator
C.the system is controlled automatically
D.all the above

A.the capstan
B.the collector
C.the panels
D.the surface

A.distribution boards
B.section boards
C.emergency switch boards
D.main switch boards