Which of the following expressions is correct in case of a CsCl unit cell (edge length 'a')?

A

$$r_{Cs^+} + r_{Cl^-} = \frac{a}{2}$$

B

$$r_{Cs^+} + r_{Cl^-} = \frac{\sqrt{3}}{2}a$$

C

$$r_{Cs^+} + r_{Cl^-} = \frac{a}{\sqrt{2}}$$

D

$$r_{Cs^+} + r_{Cl^-} = a$$