Sum of n terms of series $\;\frac{1}{1^3}\;+\frac{1+2}{1^3+2^3}\;+\frac{1+2+3}{1^3+2^3+3^3}\;+....\;is$
$(a)\;\frac{n}{n+1}\qquad(b)\;\frac{2n}{n+1}\qquad(c)\;\frac{n}{2(n+1)}\qquad(d)\;\frac{2}{n(n+1)}$