A bullet is fired normally towards an immovable wooden block. It loses 25% of its kinetic energy in penetrating through a thickness 'x' of the plank. The total thickness penetrated by the bullet into the block is :

Question

$\begin {array} {1 1} (1)\;4x & \quad (2)\;6x \\ (3)\;8x & \quad (4)\;2x \end {array}$