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

explanation:

since this is put under health, i would define "ethical (medical) behavior" the way the oath for doctors is recited.   "first do no harm."

but the term in general means when given a choice between right and wrong, we should always choose right. if we do, then we are practicing ethical behavior.

because natural gas is colorless, odorless, and tasteless, distributors add mercaptan (a chemical that smells like sulfur) to give natural gas a distinct unpleasant odor (it smells like rotten eggs). they locate the types of rock that are likely to contain natural gas deposits.