Two coaxial discs,having moments of inertia $I_1$ and $\frac{I_1}{2}$,are rotating with angular velocities $\omega_1$ and $\frac{\omega_1}{2}$ respectively,about their common axis. They are brought into contact with each other and thereafter they rotate with a common angular velocity. If $E_f$ and $E_i$ are the final and initial total energies,then $(E_f - E_i)$ is

• A
  $\frac{I_1 \omega_1^2}{6}$
• B
  $\frac{3}{8} I_1 \omega_1^2$
• C
  $-\frac{I_1 \omega_1^2}{12}$
• D
  $-\frac{I_1 \omega_1^2}{24}$