The correct relationship between unit cell edge length '$a$' and radius of sphere '$r$' for face-centred and body-centred cubic structures respectively are:

• A
  $r = 2 \sqrt{2} a$ and $\sqrt{3} r = 4 a$
• B
  $r = 2 \sqrt{2} a$ and $4 r = \sqrt{3} a$
• C
  $2 \sqrt{2} r = a$ and $4 r = \sqrt{3} a$
• D
  $2 \sqrt{2} r = a$ and $\sqrt{3} r = 4 a$