A rifle bullets loses $\left(\frac{1}{20}\right)^{th}$ of its velocity in passing through a plank. Assuming that the plank exerts a constant retarding force, the least number of such planks required just to stop the bullet is .............

  • A

    $11$

  • B

    $20$

  • C

    $21$

  • D

    Infinite

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