A round disc of moment of inertia $I_2$ about its axis perpendicular to its plane and passing through its centre is placed over another disc of moment of inertia $I_1$ rotating with an angular velocity $\omega$ about the same axis. The final angular velocity of the combination of discs is :-

Question

( A ) $\frac{ I_{1}\omega} {I_1+I_2}$
( B ) $\omega$
( C ) $\frac{I_{2}\omega}{I_{1}+I_{2}}$
( D ) $\frac{(I_{1}+I_{2})\omega}{I_1}$