The systems of the Earth have both internal and external sources of energy. Which of the following are internal sources of Earth's energy? I. fusion reactions between hydrogen atoms in Earth's core II. the decay of Earth's radioactive isotopes III. the gravitational energy from Earth's original formation IV. heat that is generated when water molecules spontaneously split in Earth's oceans A. II and IV only B. I and IV only C. II and III only D. I, II, and III only