Correct Answer: B

Rationale: Oxygen is essential for human respiration as it is the key element required for cellular respiration — the process by which cells produce energy in the form of adenosine triphosphate (ATP). When we inhale, oxygen enters the lungs, diffuses into the bloodstream, and binds to hemoglobin in red blood cells for transport throughout the body. In mitochondria, oxygen acts as the final electron acceptor in the electron transport chain, combining with hydrogen to form water and releasing energy. Without sufficient oxygen, cells cannot function efficiently, leading to hypoxia and eventual organ failure. Thus, oxygen sustains all aerobic life processes.

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