Answer:
A. Glucose
Explanation:
When oxygen is not present to serve as a terminal electron acceptor to perform aerobic respiration for energy production, yeast cells carry out ethanol fermentation. Ethanol fermentation is an incomplete oxidation of glucose in the absence of oxygen. It obtains ethanol, CO2 and a small amount of energy to the yeast cell. Glucose is broken down into pyruvate by glycolysis. Decarboxylation of pyruvate forms acetaldehyde which in turn is reduced into ethanol accompanied by regeneration of NADH.