If a passenger coach plunges into deep water (rivers, lakes, or canals), why is it impossible to open the passenger doors during the initial sinking phase, and how should occupants escape?

1. External water pressure against the doors is far greater than internal cabin air pressure; occupants should immediately break side window corners with safety hammers or push open roof escape hatches before the bus is fully submerged
2. Doors are welded shut automatically by water sensors; occupants must wait for scuba divers to cut the chassis
3. Water pressure makes windows unbreakable; passengers should wait until the bus reaches the riverbed and then open doors easily
4. Occupants should drink all incoming water to lower the water level inside the bus