The figure above shows the schematic of an automobile having a mass of 900 kg and the suspension spring constant of 81x104 N/m. If it travels at a speed of 72 km/hr on a rough road with periodic waviness as shown, what is the forcing frequency of the road on the wheel?
(a) 10 Hz (b) 4 Hz
(c) 1.5 Hz (d) 20 Hz