Four statements are given ($A$ is mass number) :
$A$. The volume of a nucleus is proportional to $A^{1/3}$.
$B$. The volume of a nucleus is proportional to $A$.
$C$. The difference in mass of an atom and its nucleus is called the mass defect.
$D$. The difference in mass of a nucleus and its constituents is called the mass defect.
Choose the correct answer from the options given below :

• A
  $A$ and $D$ are true, but $B$ and $C$ are false
• B
  $B$ and $D$ are true, but $A$ and $C$ are false
• C
  $B$ and $C$ are true, but $A$ and $D$ are false
• D
  $A$ and $C$ are true, but $B$ and $D$ are false