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

(a) rCs+ + rCl- = 3a

(b)  rCs+ + rCl- = 3a/2

(c)  rCs+ + rCl- = ($\sqrt{3}$/2)*a

(d) rCs+ + rCl-$\sqrt{3a}$