Lung volumes reflect obstructive disease severity Steps:

• Identify the graph as showing forced vital capacity (FVC), measuring lung function after deep inhalation.
• Compare volumes: X highest, Z medium, Y lowest, indicating decreasing expiratory efficiency.
• Recall obstructive diseases like chronic bronchitis and emphysema reduce FVC by narrowing airways.
• Match order: normal (X), chronic bronchitis (Z), emphysema (Y) explains the gradient.

Why C is correct:

• X Z Y orders people from highest to lowest volume, aligning with normal function, bronchitis, and emphysema severity per pulmonary function definitions.

Why the others are wrong:

• A lists conditions without specifying volume order or person assignment.
• B (Y X Z) mismatches graph's decreasing volume sequence starting with Y lowest.
• D (Z Y X) reverses the observed high-to-low volume trend.

Final answer: C