Choose the incorrect statement from the following.

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 :

The nearest distance between two atoms in case of a $bcc$ lattice is equal to

The radius of a nucleus with atomic number $Z = 50$ is $9 \times 10^{-13} \ m$. The electric potential at its surface is:

The atoms of the same element having different masses but same chemical properties are called:

Mass numbers of two nuclei are in the ratio of $4: 3$. Their nuclear densities will be in the ratio of