Radiant Energy Reaching the Earth from the Sun Is Not

Radiant energy reaching the Earth from the sun is not sufficient to keep the Earth's surface temperature comfortable for life. Of the following, the most likely source of the additional thermal energy is from

A) fusion of hydrogen nuclei in the Earth's core.
B) fusion of high Z nuclei in the Earth's core.
C) fission of radioactive nuclei inside the Earth.
D) fission of radioactive nuclei in cosmic rays.
E) deflection of radioactive nuclei in cosmic rays.