The signal supplied by a transmitter fitted with a 3-phase AC generator, connected to RPM indicator, is:

A、anACvoltagevaryingwiththeRPM;theindicatorrectifiesthesignalviaadiodebridgeandisprovidedwithavoltmeter
B、aDCvoltagevaryingwiththeRPM;theindicatorisaplainvoltmeterwitharev/min.scale
C、anACvoltage,thefrequencyofwhichvarieswiththeRPM;theindicatorconvertsthesignalintosquarepulseswhicharethencounted
D、athree-phasevoltage,thefrequencyofwhichvarieswiththeRPM;theindicatorisprovidedwithamotorwhichdrivesamagnetictachometer