B, the ball moves in a circular motion, and the motion state changes constantly, so the ball is bound to be affected by unbalanced force, so B is correct;
C, when the ball leaves the finger, because the inertial ball continues to move, the force exerted by the finger on the ball does not do work on the ball at this time, so c is wrong;
D, because the ball is in weightlessness, it is only pulled by the rope when it rotates, so D is correct.
So choose C.