11. A hole should be made in the sagged tube occurring in a water-tube boiler, prior to plugging the tube to prevent a __________.

Drilling a hole in a sagged tube before plugging prevents pressure buildup. Plugging a sagged tube creates a sealed volume; without a vent, trapped fluids can increase pressure and risk rupture. This practice avoids the hazard of overpressure, not further sagging, burnout, or crack failure.