Identify vein and calculate lumen area Steps:

• Capillary Q has thinnest wall (<1 µm) and smallest lumen (0.008 mm).
• Artery R has thickest wall (1000 µm) for high pressure.
• Vein P has intermediate wall (500 µm) and largest lumen (5 mm).
• Area = π × (diameter/2)² = 3.14 × (5/2)² = 3.14 × 6.25 = 19.6 mm².

Why B is correct:

• Matches π r² formula for P's 5 mm diameter lumen (r = 2.5 mm).

Why the others are wrong:

• A: Underestimates area using diameter instead of radius.
• C: Approximates without π, ignoring circular shape.
• D: Uses full diameter as radius, doubling the value.

Final answer: B