Through the first three stages of cellular respiration only 4 ATP molecules have been produced from the initial glucose molecule. In which of the products of these stages is the potential energy to produce more ATP molecules stored?

a) The 6 molecules of CO₂.

b) The H⁺ ions produced.

c) The 10 NADH and 2 FADH₂ molecules.

d) The 4 molecules of ATP.