If $a$ stands for the edge length of the cubic systems: simple cubic,body-centered cubic,and face-centered 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
  $1a : \sqrt{3}a : \sqrt{2}a$
• C
  $\frac{1}{2}a : \frac{\sqrt{3}}{4}a : \frac{1}{2\sqrt{2}}a$
• D
  $\frac{1}{2}a : \sqrt{3}a : \frac{1}{\sqrt{2}}a$