The answer is; C
The radioisotopes in the interior of the earth are not in pure form and have many embedded impurities and are interspersed. Therefore when a neutron is released by a decaying atom, its chances of hitting another radioisotope atom (to continue the chain reaction) are lower than in a manmade nuclear reactor. This way, the radioactive chain reaction in the earth’s interior is not well sustained.
The graphs can be described as
So in case of inverse relationship we will obtain a curve not straight line
To calculate electronegativity, start by going online to find an electronegativity table. You can then assess the quality of a bond between 2 atoms by looking up their electronegativities on the table and subtracting the smaller one from the larger one. If the difference is less than 0.5, the bond is nonpolar covalent.