$A$ bullet of mass $m$ enters a wooden block of length $L$ at a speed $v_1$ and emerges out of the block with a speed $v_2$. If $F$ is the average force which impeded its motion through the wooden block, then (Assume uniform deceleration inside the block):

• A
  $F = \frac{m}{2L}(v_1^2 - v_2^2)$
• B
  $F = \frac{m}{2L}(v_2^2 - v_1^2)$
• C
  $F = \frac{m}{2L}(v_1^2 + v_2^2)$
• D
  $F = \frac{m}{L}(v_1^2 - v_2^2)$