20. A uniform plate of mass M is placed on two fast-rotating identical wheels of radius r. The coefficients of kinetic friction between the plate and both wheels are μ, the distance between the centers of the two wheels is , the wheel on the right (left) rotates counterclockwise (clockwise) with angular speed ω (see Fig. 5), and the gravitational acceleration is denoted g. If the mass center of the plate is displaced by d from the center of the two wheels, what is the frequency the plate oscillates with? (Assuming the plate is so heavy that it always stays on the wheels.)

(A) ω
(B)
(C)
(D)