$A$ thin stiff insulated metal wire is bent into a circular loop with its two ends extending tangentially from the same point of the loop. The wire loop has mass $m$ and radius $r$ and it is in a uniform vertical magnetic field $B_0$,as shown in the figure. Initially,it hangs vertically downwards,because of acceleration due to gravity $g$,on two conducting supports at $P$ and $Q$. When a current $I$ is passed through the loop,the loop turns about the line $PQ$ by an angle $\theta$ given by

• A
  $\tan \theta = \frac{\pi r I B_0}{mg}$
• B
  $\tan \theta = \frac{2 \pi r I B_0}{mg}$
• C
  $\tan \theta = \frac{\pi r I B_0}{2 mg}$
• D
  $\tan \theta = \frac{mg}{\pi r I B_0}$