What is the safety rule regarding center aisles and emergency exit pathways inside a commercial passenger bus?

1. Passengers may place folding chairs in aisles if all regular seats are taken
2. Large suitcases may be placed in front of emergency doors if luggage bays are full
3. Standing passengers may stack backpacks in the center aisle to create seating
4. Center aisles, emergency exit doors, and escape pathways must remain completely clear and unobstructed at all times