During cellular respiration, particularly during the glycolysis stage where pyruvic acid is generated, in addition to the production of ATP, two molecules of NADH are also produced. However, from the options you've provided, the best response that relates to other by-products or results of cellular metabolism (not necessarily specific to the formation of pyruvic acid) would be heat, since energy is often released as heat during metabolic reactions.
So, the answer is:
heat
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