A metal wire of length l and area of cross section A is subjected to a longitudinal force Mg. The wire elongates by a length l. If the Youngs Modulus of the material of the wire is Y, the work done by the external force is:

A

$$ \frac{A}{2L} Y1^{2} $$

B

$$Mgl$$

C

$$( \frac{Mg}{A^{2}}) l $$

D

$$ \frac{YAl^{2}}{L} $$