$A$ frictionless pulley is attached to one arm of a balance and a string passed around it carries two masses $m_1$ and $m_2$. The pulley is provided with a clamp due to which $m_1$ and $m_2$ do not move with respect to each other. On removing the clamp,if the counter mass restores balance,then the acceleration of the centre of mass of the masses $m_1$ and $m_2$ will have a magnitude of?

• A
  zero
• B
  $\left( \frac{m_1 - m_2}{m_1 + m_2} \right)g$
• C
  $\left( \frac{m_1 - m_2}{m_1 + m_2} \right)^2 g$
• D
  $g$