As shown in the figure, a bob of mass m is tied to a massless string whose other end portion is wound on a fly wheel (disc) of radius r and mass m. When released from rest the bob starts falling vertically. When it has covered a distance of h, the angular speed of the wheel will be:

A

$$\frac{1}{r}\sqrt{\frac{4gh}{3}}$$

B

$$r\sqrt{\frac{3}{2gh}}$$

C

$$\frac{1}{r}\sqrt{\frac{2gh}{3}}$$

D

$$r\sqrt{\frac{3}{4gh}}$$