Gas Proportions in Inspired and Expired Air

Steps:

• Identify largest segment as nitrogen, which comprises ~78% in both inspired and expired air and remains unchanged.
• Assign second-largest segment to oxygen, ~21% in inspired air decreasing to ~16% in expired air.
• Label smallest segment as other gases, including trace CO2 (~0.04% inspired, ~4% expired) and water vapor.
• Match segments to choices based on relative sizes: 1 (largest, N2), 2 (medium, O2), 3 (smallest, others).

Why B is correct:

• Nitrogen dominates at ~78%, oxygen is next at 16-21%, and other gases (CO2, etc.) are minor, aligning with segment sizes per respiratory physiology.

Why the others are wrong:

• A: Misplaces other gases as medium segment, but they are smallest.
• C: Erroneously assigns oxygen to largest segment, ignoring nitrogen's dominance.
• D: Incorrectly labels smallest segment as only CO2, excluding other trace gases.

Final answer: B