The mass density of a nucleus varies with mass number $A$ as

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$: The nuclear density of nuclides ${ }_{5}^{10} B, { }_{3}^{6} Li, { }_{26}^{56} Fe, { }_{10}^{20} Ne$ and ${ }_{83}^{209} Bi$ can be arranged as $\rho_{ Bi }^{ N } > \rho_{ Fe }^{ N } > \rho_{ Ne }^{ N } > \rho_{ B }^{ N } > \rho_{ Li }^{ N }$.
Reason $R$: The radius $R$ of a nucleus is related to its mass number $A$ as $R = R_0 A^{1/3}$,where $R_0$ is a constant.
In the light of the above statements,choose the correct answer from the options given below:

Show that the volume of a nucleus is proportional to its atomic mass number $A$.

If the nuclear force between two protons,two neutrons,and between a proton and a neutron is denoted by $F_{pp}$,$F_{nn}$,and $F_{pn}$ respectively,then:

Which of the following pairs is an isobar?

Assertion: Neutrons penetrate matter more readily as compared to protons.
Reason: Neutrons are slightly more massive than protons.