An isolated turntable rotates at a constant angular velocity of 8.0rad/s. Which statement best describes the net external torque acting on the turntable?
a) A constant non-zero external torque must act in the direction of rotation to keep the disk spinning at 8.0rad/s.
b) A constant external torque acts opposite the direction of rotation to cancel out frictional forces.
c) The net external torque is zero; therefore the turntable's angular velocity remains constant.
d) The external torque continuously changes direction during each revolution, so its average value over time is zero.