$A$ nucleus has mass number $A_1$ and volume $V_1$. Another nucleus has mass number $A_2$ and volume $V_2$. If the relation between the mass numbers is $A_2 = 3 A_1$,then $\frac{V_1}{V_2} =$

• A
  $3^{1/3}$
• B
  $(1/3)^{1/3}$
• C
  $1/3$
• D
  $1/\sqrt{3}$