A horizontal disc rotates freely about a vertical axis through its centre. A ring, having the same mass and radius as the disc , is now gently placed on the disc. After some time, the two rotate with a common angular velocity. 1) some friction exits between the disc and the ring 2) the angular mometum of disc plus ring is conserved. 3) the final common angular velocity is $\large\frac{2}{3}$ rd of the initial angular velocity of disc. 4) $\large\frac{2}{3}$ rd of the initial kinetic energy is converted to heat. Which of the following is correct?

Question

a) 2 , 3 and 4

b) 1 , 2 and 4

c) 1,2 and 3

d) 1,3 and 4