Why is large-scale nuclear fission a rare phenomenon on Earth?

Large-scale nuclear is fission a rare phenomenon on Earth because

atoms that undergo fission aren't concentrated enough to sustain a chain reaction.

Atoms that undergo fission aren't concentrated enough to sustain a chain reaction. For example, the decay chain of U-238 is called the uranium series. ... Uranium is find in ores on Earth with very law mass percentage in it, so fission is not possible

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.