When a vehicle travels on a rough road whose undulations can be assumed to be sinusoidal, the resonant conditions of the base – excited vibrations, are determined by the
(a) Mass of the vehicle, stiffness of the suspension spring, speed of the vehical, wavelength of the roughness curve
(b) Speed of the vehicle only
(c) Speed of the vehicle and the stiffness of the suspension spring
(d) Amplitude of the undulations