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A horizontal disc rotating freely about a vertical axis through its centre makes 90 revolutions per minute. A small piece of wax of mass \( \boldsymbol{m} \) falls vertically on the disc and sticks to it at a distance \( r \) from the axis. If the number of revolutions per minute reduce to 60 , then the moment of inertia of the disc is
(1) \( m r^{2} \)
(2) \( \frac{3}{2} m r^{2} \)
(3) \( 2 m r^{2} \)
(4) \( 3 m r^{2} \)
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