Answer:
B. O2 is reduced when it accepts electrons and forms water.
Step-by-step explanation:
During cellular respiration water is formed when oxygen receives electrons and picks up protons at the conclusion of the electron transport chain. The electron transport chain will stop functioning if there isn't enough oxygen to receive electrons (for example, because a person isn't breathing in enough oxygen), and ATP will no longer be created by chemiosmosis. Cells can't carry out the reactions they need to function if they don't even have enough ATP, and they may perish after a long period of time.