Without adequate oxygen, the body's cells:

• A. Cease metabolism altogether, resulting in carbon dioxide accumulation in the blood.
• B. Begin to metabolize fat, resulting in the production and accumulation of ketoacids.
• C. Incompletely convert glucose into energy, and lactic acid accumulates in the blood.
• D. Rely solely on glucose, which is completely converted into adenosine triphosphate.

Answer: (C)

When oxygen is limited, cells can’t run the full aerobic respiration that makes most ATP. Instead, they keep glycolysis going in the cytoplasm, which breaks glucose down to pyruvate and yields only a small amount of ATP. To keep glycolysis going without oxygen, pyruvate is converted to lactate, regenerating NAD+ so glycolysis can continue. This causes lactic acid to accumulate in the blood, signaling anaerobic metabolism.

That’s why this option is best: it describes incomplete glucose breakdown with lactic acid buildup, which is the hallmark of cells operating under low-oxygen conditions. The other possibilities aren’t consistent with how cells meet energy needs without oxygen—metabolism doesn’t stop, fat is not the primary fuel turned into ketoacids in this scenario, and glucose isn’t perfectly converted into ATP just by using oxygen-starved pathways.