$A$ thin string is wrapped around the circumference of a wheel of radius $r$. The wheel has a horizontal axle and a moment of inertia $I$ about it. $A$ weight $mg$ is attached to the end of the string,which falls from rest. After falling through a distance $h$,the angular velocity of the wheel will be:

• A
  $\sqrt{\frac{2gh}{I + mr}}$
• B
  $\left[\frac{2mgh}{I + mr^2}\right]^{1/2}$
• C
  $\left[\frac{2mgh}{I + 2mr^2}\right]^{1/2}$
• D
  $\sqrt{2gh}$