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

Question

( A ) $r_{_{Cs^+}} + r_{_{Cl^-}} = {3a}$
( B ) $r_{_{Cs^+}} + r_{_{Cl^-}} = \frac{3a}{2}$
( C ) $r_{_{Cs^+}} + r_{_{Cl^-}} = {\sqrt{3}} a$
( D ) $r_{_{Cs^+}} + r_{_{Cl^-}} = \frac{\sqrt{3}}{2} a$