$A$ block of mass $m$,attached to a spring of spring constant $k$,oscillates on a smooth horizontal table. The other end of the spring is fixed to a wall. The block has a speed $v$ when the spring is at its natural length. Before coming to an instantaneous rest,if the block moves a distance $x$ from the mean position,then

• A
  $x = \sqrt{m/k}$
• B
  $x = \frac{1}{v} \sqrt{m/k}$
• C
  $x = v \sqrt{m/k}$
• D
  $x = \sqrt{mv/k}$