Total volume of atoms present in a face-centred cubic unit cell of a metal is ($r$ is atomic radius)

• A
  $\frac{12}{3} \pi r^3$
• B
  $\frac{16}{3} \pi r^3$
• C
  $\frac{20}{3} \pi r^3$
• D
  $\frac{24}{3} \pi r^3$