• Classification of Thick vs. Thin Shell Pressure Vessels:
• In structural mechanics, cylindrical pressure vessels are classified by the ratio of internal diameter to wall thickness ($D/t$):
• Thin shells: $D/t \ge 10 - 20$, where radial stress is negligible and membrane hoop stress is uniform.
• Thick shells: Lamé's equations apply due to pronounced non-uniform stress distribution across the thick wall.
• Official Answer Key: Designated Option 2 (>10) as the credited option.
• Conclusion: >10 (Option 2).

Correct option: (2) >10