# A steel wire of $40\;m$ in length is stretched through $2.0\; mm.$ The cross sectional area of the wire is $2.0 \;mm^2$ . If young's modulus of steel is $2.0 \times 10^{11}\;N/m^2$ the energy density of wire is
$\begin {array} {1 1} (a)\;2.5 \times 10^4 J/m^3 \\ (b)\;5.0 \times 10^4 J/m^3 \\ (c)\;7.5 \times 10^4 J/m^3 \\ (d)\;10.0 \times 10^4 J/m^3 \end {array}$