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

• A

  ${\mathrm{r}}_{{\mathrm{Cs}}^{+}}+{\mathrm{r}}_{{\mathrm{Cl}}^{-}}=3\mathrm{a}$

• B

  ${\mathrm{r}}_{{\mathrm{Cs}}^{+}}+{\mathrm{r}}_{{\mathrm{Cl}}^{-}}=\frac{3\mathrm{a}}{2}$

• C

  ${\mathrm{r}}_{{\mathrm{Cs}}^{+}}+{\mathrm{r}}_{{\mathrm{Cl}}^{-}}=\frac{\sqrt{3}}{2}\mathrm{a}$

• D

  ${\mathrm{r}}_{\mathrm{Cs}}+{\mathrm{r}}_{{\mathrm{Cl}}^{-}}=\sqrt{3}\mathrm{a}$