Answer: A. Radioactivity
Explanation: Internal heat is a feature of planets and other celestial bodies, such as stars and others, and according to some scientific claims, even some of the asteroids had internal heat in the early stages of the solar system.
The most important internal heat source of the earth and the other planets is certainly radioactivity, which means that the natural radioactive decay of radioactive elements in the earth releases heat, which as such is the most significant internal heat source. The reaction of radioactive decay emits heat; more precisely, the radiation energy of radioactive particles induces the movement of atoms and the increase of their energy, and this increase in energy means heat.
It is known that gravitational force acts not only on earth but also on other celestial bodies, so this force is also an important source of the inner heat of celestial bodies. There are also geothermal energy sources on earth, and many others mentioned.
