A uniform metal rod (fixed at both ends) of 2 ${\mathrm{mm}}^2$ crosssection is cooled from 40° C to 20° C. The co-efficient of the linear expansion of the rod is  12 x ${10}^{-6}$ per degree and its young modulus of elasticity is ${10}^{11} \mathrm{N}/{\mathrm{m}}^2$. The energy stored per unit volume of the rod is:

• A

  $2880 \mathrm{J}/{\mathrm{m}}^3$

• B

  $1500 \mathrm{J}/{\mathrm{m}}^3$

• C

  $5760 \mathrm{J}/{\mathrm{m}}^3$

• D

  $1440 \mathrm{J}/{\mathrm{m}}^3$