A solid $A^{+}B^{-}$ has an $NaCl$ type closet packed structure . If the radius of the anion is $250\;pm$ What is the radius of the cation ?

Question

$\begin{array}{1 1} 103.5\;pm \\ 7 \;pm \\ 8.4\;pm \\ 9.5\;pm\end{array}$