A bullet of mass m moving with a horizontal velocity u strikes a stationary wooden block of mass M suspended by a string of length L = 50 cm. The bullet emerges out of the block with speed $\frac{u}{4}$. If  M = 6 m, the minimum value of u, so that the block can complete the vertical circle, is (Take, $\theta$ = 10 $m{s}^{-2}$)

• A

  10 $m{s}^{-1}$

• B

  20 $m{s}^{-1}$

• C

  30 $m{s}^{-1}$

• D

  40 $m{s}^{-1}$