The cross-section of a member is subjected to a uniform shear. The strain energy stored per unit volume is equal to (G = modulus of rigidity)

A

$$\frac{2 \tau^{2}}{G}$$

B

$$\frac{ \tau^{2}}{2G}$$

C

$$\frac{ \tau^{2}}{4G}$$

D

$$\frac{ \tau^{2}}{G}$$