Two discs of moments of inertia $I_{1}$ and $I_{2}$ about their respective axes, rotating with angular frequencies $\omega_{1}$ and $\omega_{2}$ respectively, are brought into contact face to face with their axes of rotation coincident. The angular frequency of the composite disc will be
(a) $\frac{I_{1} \omega_{1}+I_{2} \omega_{2}}{I_{1}+I_{2}}$
(b) $\frac{I_{2} \omega_{1}+I_{1} \omega_{2}}{I_{1}+I_{2}}$
(c) $\frac{I_{1} \omega_{1}-I_{2} \omega_{2}}{I_{1}-I_{2}}$
(d) $\frac{I_{2} \omega_{1}-I_{1} \omega_{2}}{I_{1}-I_{2}}$