The internal sources of Earth's energy include processes that originate from within the Earth itself. Let's analyze each option:
I. Fusion reactions between hydrogen atoms in Earth's core - This is not accurate, as Earth's core does not undergo fusion reactions like the Sun does. So, this option is not considered an internal source of energy.
II. The decay of Earth's radioactive isotopes - This is indeed an internal source of energy, as radioactive decay occurs within the Earth and contributes to its heat.
III. The gravitational energy from Earth's original formation - This is also considered an internal source, as the energy from the gravitational pull during Earth's formation contributes to its current internal heat.
IV. Heat that is generated when water molecules spontaneously split in Earth's oceans - This process, known as water splitting, does not significantly contribute to the Earth's internal energy, as it is mainly a surface process driven by external energy sources (like sunlight).
Based on the analysis:
- Internal sources of Earth's energy are II (radioactive decay) and III (gravitational energy from original formation).
Thus, the correct answer is:
A. II and III only.