What is the MOST probable heat source that produced extensive,maybe even total,melting of the Moon at an early stage in its history?

Question

Quizplus · Accepted Answer

The most probable heat source that produced extensive melting of the Moon at an early stage in its history is the decay of radioactive elements within it and the impact energy of meteoritic bombardment. This is supported by evidence of the extensive volcanic activity on the Moon in its early history, which is indicative of a high amount of internal heat. The decay of radioactive elements would have contributed to this heat, while meteoritic bombardment would have caused the melting and resurfacing of large areas of the Moon's surface.

What Is the MOST Probable Heat Source That Produced Extensive,maybe