$A$ bullet of mass $m$ strikes a pendulum-bob of mass $M$ with velocity $u$. It passes through it and emerges out with a velocity $u/2$ from the bob. The length of the pendulum is $\ell$. What should be the minimum value of $u$,if the pendulum-bob will swing through a complete circle?

• A
  $\frac{2 M}{m} \sqrt{5 g \ell}$
• B
  $\frac{M}{2 m} \sqrt{5 g \ell}$
• C
  $\frac{2 M}{m} \frac{1}{\sqrt{5 g \ell}}$
• D
  $\frac{M}{2 m} \frac{1}{\sqrt{5 g \ell}}$