If the sum $\; \large\frac{3}{1^{2}} + \large\frac{5}{1^{2} + 2^{2}} + \large\frac{7}{1^{2} + 2^{2} +3^{2}}+----+\;$ upto 20 terms is equal to $\;\large\frac{k}{21}\;$, then k is equal to :

Question

$(a)\;120\qquad(b)\;180\qquad(c)\;240\qquad(d)\;60$