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

Question

$(a)\;r_{Cs}+r_{Cl^{-}} =3a \qquad(b)\; r_{Cs}+r_{Cl^{-}} =\large\frac{3a}{2}\qquad(c)\;r_{Cs}+r_{Cl^{-}} = \large\frac{\sqrt{3}}{2}a\qquad(d)\;r_{Cs}+r_{Cl^{-}} =\sqrt{3}a$