A long wire hangs vertically with its upper end clamped. A torque of 8 Nm applied to the free end twists it through 45^o. The potential energy of the twisted wire is 

• A

  $\mathrm{\pi J}$

• B

  $\frac{\mathrm{\pi }}{2}\mathrm{J}$

• C

  $\frac{\mathrm{\pi }}{4}\mathrm{J}$

• D

  $\frac{\mathrm{\pi }}{8}\mathrm{J}$