A spring of force constant k is stretched by a small length ' x .The work done in stretching a further by a small length 'y' is (A) `(1)/(2)k(x^(2)+y^(2))` (B)`(1)/(2)k(x+y)^(2)` (C)`(1)/(2)k(y^(2)+x^(2))`(D) `(1)/(2)ky(2x+y)`