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

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.