The correct phase rotation of a three-phase alternator can be checked with a（）

A.loss of field excitation
B.total loss of load
C.large increase in load
D.loss of steam pressure

A.temperature rise
B.input kilowatts
C.horsepower
D.operating voltage

A.When it is a DC machine
B.When it is an AC machine
C.Any machine that operates on over 100 volts
D.Only when working on wet steel plates