$A$ thin circular disc of mass $M$ and radius $R$ is rotating in a horizontal plane about an axis passing through its centre and perpendicular to its plane with angular velocity $\omega$. If another disc of same dimensions but of mass $M/2$ is placed gently on the first disc co-axially,then the new angular velocity of the system is:

• A
  $\frac{4}{5} \omega$
• B
  $\frac{5}{4} \omega$
• C
  $\frac{2}{3} \omega$
• D
  $\frac{3}{2} \omega$