$A$ small block of mass $m$ is placed on a wedge of mass $M$ as shown,which is initially at rest. All the surfaces are frictionless. The spring attached to the other end of the wedge has a force constant $k$. If $a'$ is the acceleration of $m$ relative to the wedge as it starts coming down and $A$ is the acceleration acquired by the wedge as the block starts coming down,then the maximum velocity of $M$ is:

• A
  $\sqrt {2gh} $
• B
  $\sqrt {\frac{{2ghm}}{{m + M}}} $
• C
  $\sqrt {\frac{{2{m^2}gh}}{{mM + {M^2}}}} $
• D
  None