In Fig. $4.20$ shown here, pulley and spring are ideal. If $k$ is spring constant of spring, the potential energy stored in it is $\left(m_{1}>m_{2}\right)$
Fig. $4.20$
(A) $\frac{2 m_{1}^{2} g^{2}}{k}$
(B) $\frac{2 m_{2}^{2} g^{2}}{k}$
(C) $\frac{\left(m_{1}+m_{2}\right)^{2} g^{2}}{k}$
(D) $\frac{1}{2} \frac{\left(m_{1}-m_{2}\right)^{2} g^{2}}{k}$