Explanation

(c) Let initial velocity of body at point $A$ is $v,\ AB\ $is 3 cm.
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From $v^{2} = u^{2} - 2as$
$\left( \frac{v}{2} \right)^{2} = v^{2} - 2a \times 3$
$a = \frac{v^{2}}{8}$
Let on penetrating 3 cm in a wooden block, the body moves $x$ distance form $B\ to\ C.$
So, for $B\ to\ C$
$\ \ \ \ \ \ \ \ \ u = \frac{v}{2},v = 0,$
$\ \ \ \ \ \ \ \ \ \ s = x,\ a = \frac{v^{2}}{8}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (deceleration)$
$\therefore(0)^{2} = \left( \frac{v}{2} \right)^{2} - 2\ .\frac{v^{2}}{8}\ .\ x$
$\ \ \ \ \ \ \ \ \ x = 1$