A thin uniform 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 $\mathrm{\omega }$: Another disc of same dimensions 

but of mass M/4 is placed gently on the first disc co-axially. The angular velocity $\mathrm{\omega }$'of the system is

• A

  $\sqrt{2}\mathrm{\omega }$

• B

  $(4/5)\mathrm{\omega }$

• C

  $\text{ (3/4) }\mathrm{\omega }$

• D

  $(1/3)\mathrm{\omega }$