$A$ horizontal disc rotating freely about a vertical axis through its centre makes $90$ revolutions per minute. $A$ small piece of wax of mass $m$ falls vertically on the disc and sticks to it at a distance $r$ from the axis. If the number of revolutions per minute reduces to $60$,then the moment of inertia of the disc is .........

• A
  $m r^2$
• B
  $\frac{3}{2} m r^2$
• C
  $2 m r^2$
• D
  $3 m r^2$