Calculate the overall complex dissociation equilibrium constant for the $Cu(NH_3) _4^{2+}$ ion given that $B_4$ for this complex is $2:1 \times 10^{13}$

Question

$\begin{array}{1 1} (a)\;8.7 \times 10^{-14} \\ (b)\;5.7 \times 10^{-14} \\ (c)\;4.7 \times 10^{-14} \\(d)\;3.7 \times 10^{-14} \end{array}$