A copper wire (y = 1 x 10¹¹ n/m²) of length 8 m, and a steel wire (y = 2 x 10¹¹ n/m²) of the length 4m, each of cross - section 0.5 cm² are fastened end to end and stretched with a tension of 500 n. the elastic potential energy of the system is

a.0.125 j
b.0.2 j
c.0.25 j
d.0.5 j