$A$ thin circular ring of mass $M$ and radius $R$ is rotating in a horizontal plane with angular velocity $\omega$ about an axis passing through its center and perpendicular to the plane. If another ring of the same size but mass $\frac{M}{4}$ is placed coaxially on the first ring,then the new angular velocity of the system is:

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