CsCl crystallises in body centred cubic lattice. If 'a' is its edge length then which of the following expressions is correct ?

Question

$\begin{array}{1 1}(A)\;r_{Cs^+}+r_{Cl^-}=\large\frac{\sqrt 3}{2}\normalsize a\\(B)\;r_{Cs}+r_{Cl^-}=\sqrt 3a\\(C)\;r_{Cs}+r_{Cl^-}=3a\\(D)\;r_{Cs^+}+r_{Cl^-}=\large\frac{\ 3a}{2}\end{array}$