A.) In biological nitrogen fixation, (hydrogen, oxygen, N2, or ammonia) is (oxidized or reduced) when it accepts (hydrogen, oxygen, or nitrogen) atoms, resulting in the production of (water, nitrogen gas, nitrate, or ammonia).
B.) Legumes produce (nitrogenase, nitrogen, leghemoglobin, oxygen, or bacteria) which regulates the amount of (nitrogen, leghemoglobin, oxygen, bacteria, or nitrogenase) present in the root nodule so (nitrogenase, nitrogen, leghemoglobin, oxygen, or bacteria) activity is not inhibited.
C.) The nitrogen sources used by humans are (inorganic or organic) compounds, while nitrogen gas is an (organic or inorganic) compound.