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- = (3/2)*a
(d) rCs+ + rCl- = 3a