Where do the internal heat of earth come from

As Earth grew larger, the increased pressure on Earth's interior caused it to compress and heat up. Heat also came from friction when melted material was redistributed within Earth, forming the core and mantle. A major source of Earth's heat is radioactivity, the energy released when the unstable atoms decay.

answer; /// the answer to the first question is (b) it began when people first deliberately seeds; /// and (d); // it gave people a steady food supple for the first time;

answer; researchers typically cannot control people's sexual behaviors as would be necessary in an experiment;