The figure below shown two rotors connected by an elastic shaft undergoing torsional vibration. The rotor has a mass of 2 kg and a diameter of 60 cm, while the rotor has a mass of 1 kg and a diameter of 20cm. what is the distance l at which the node of vibration of torsonal vibration occurs?
(a) 36cm (b) 30cm
(c) 22cm (d) 18cm