Answer to Question 1:
Well, given the choices to the question above 'Lactic acid fermentation can be modeled by which equation?', I would say that the best choice would be 'Pyruvic acid + NADH → Lactic acid + NAD+', or in other words, Option D.
Explanation for Question 1:
For a further explanation expanding more on the first question and showing why, the user above's answer is super helpful.
Answer to Question 2:
For the additional question asked 'In the Krebs cycle, how is citric acid formed?', I didn't see the options, however I believe they are:
O Carbon dioxide bonds with a chain of coenzyme A.
O Pyruvic acid molecules are broken down by an acetyl group.
O Acetyl-CoA joins with a large molecule called ozaloacetic acid.
O Enzymes combine hydrogen ions, oxygen, and electrons.
^ This being said, I would say your best choice would be Option C.
Explanation for Question 2:
The Krebs cycle begins when acetyl-CoA combines with a four-carbon molecule called OAA, aka oxaloacetate. This begins to produce citric acid, in which has six carbon-atoms. This is why the Krebs cycle is also referred to as 'the citric acid cycle.'
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