What process must happen before this process depicted below?

two pyruvate molecules are converted to two lactic acid molecules. two nadh molecules are converted to two nad+ molecules in the process.

fermentation
cellular respiration
krebs cycle
glycolysis

In glycolysis, glucose molecule is converted into pyruvate molecules .

It can't be cellular respiration because it starts always with glucose i.e glycolysis. But here it is the process after glycolysis (starting from pyruvate) and changing into lactic acid. It's an aerobic respiration called as lactic acid fermentation.

Fermentation

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explanation: