Two discs are rotating about their axes, normal to the discs and passing through the centres of the discs. Disc D, has 2 kg mass and 0.2 m radius and initial angular velocity of 50 rad s. Disc D, has 4kg mass, 0.1 m radius and initial angular velocity of 200 rad s. The two discs are brought in contact face to face, with their axes of rotation coincident. The final angular velocity (in rad s) of the system is (a) 40 (c) 100 (b) 60 (d) 120