Lungs eliminate toxic metabolic waste

Steps:

• Identify CO2's origin: Produced during cellular respiration as cells break down glucose for energy.
• Recognize buildup risk: Excess CO2 in blood forms carbonic acid, lowering pH and causing acidosis.
• Understand lung function: Lungs exhale CO2 to maintain blood pH and prevent toxicity.
• Evaluate options: Match to physiological role, ruling out unrelated environmental or plant processes.

Why B is correct:

• CO2 is a metabolic waste from aerobic respiration (C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP), and high levels are poisonous, causing respiratory acidosis.

Why the others are wrong:

• A: Greenhouse effect is atmospheric, not a bodily concern.
• C: CO2 actually decreases blood pH by forming acid, not increases it.
• D: Plant photosynthesis is irrelevant to human physiology.

Final answer: B