The nucleus of an atom can be assumed to be spherical. The radius of the nucleus of mass number $A$ is given by $1.25 \times 10^{-13} \times A^{1/3} \ cm$. The radius of the atom is $1 \ \mathring{A}$. If the mass number is $64$,then the fraction of the atomic volume that is occupied by the nucleus is:

• A
  $1.0 \times 10^{-3}$
• B
  $5.0 \times 10^{-5}$
• C
  $2.5 \times 10^{-2}$
• D
  $1.25 \times 10^{-4}$