If a stands for the edge length of the cubic systems: simple cubic, body centred cubic and face centred cubic, then the ratio of radii of the spheres in these systems will be respectively
(a) \(\frac{1}{2} a: \frac{\sqrt{3}}{2} a: \frac{\sqrt{2}}{2} a\)
(b) \(1 a: \sqrt{3} a: \sqrt{2} a\)
(c) \(\frac{1}{2} a: \frac{\sqrt{3}}{4} a: \frac{1}{2 \sqrt{2}} a\)
(d) \(\frac{1}{2} a: \sqrt{3} a: \frac{1}{\sqrt{2}} a\)