An unstable heavy nucleus at rest breaks into two nuclei which move away with velocities in the ratio of $8:27$. The ratio of the radii of the nuclei (assumed to be spherical ) is

The radius of $R$ of a nucleus of mass number $A$ can be estimated by the formula $R =\left(1.3 \times 10^{-15}\right) A ^{1 / 3}\, m .$ It follows that the mass density of a nucleus is of the order of

$\left( M _{\text {prot. }} \cong M _{\text {neut. }}=1.67 \times 10^{-27} kg \right)$

Consider the following statements :

$A.$ Atoms of each element emit characterstics spectrum.

$B.$ According to Bohr's Postulate, an electron in a hydrogen atom, revolves in a certain stationary orbit.

$C.$ The density of nuclear matter depends on the size of the nucleus.

$D.$ A free neutron is stable but a free proton decay is possible.

$E.$ Radioactivity is an indication of the instability of nuclei.

Choose the correct answer from the options given below 

Choose the correct statement

The mass number of a nucleus is