Which of the following can be used for carrying out a nuclear fission reaction?

Radioactivity is due to

Nuclei tend to have more neutrons than protons at high mass numbers because

In nuclear reactors,the speed of the neutrons is slowed down by:

The isotopic mass of ${}_{3}^{7}Li$ is $7.016005 \ u$ and those of the $H$ atom and neutron are respectively $1.007825 \ u$ and $1.008665 \ u$. Then,the binding energy of the ${}_{3}^{7}Li$ nucleus is: (in $MeV$)

Which of the following statements about radioactivity of an element is incorrect?