An 8-pole wound rotor induction motor operating at 60Hz supply is driven at ___

An 8-pole wound rotor induction motor operating at 60Hz supply is driven at 1.800 rpm by a prime mover in the opposite direction of the revolving field.

The rotor current frequency is

A. 60 Hz
B. 120 Hz.
C. 180 Hz
D. none of the above