We sit in the room with windows open. Then
(A) air pressure on the floor of the room equals the atmospheric pressure but the air pressure on the ceiling is negligible
(B) air pressure is nearly the same on the floor, the walls and the ceiling
(C) air pressure on the floor equals the weight of the air column inside the room (from floor to ceiling) per unit area
(D) air pressure on the walls is zero since the weight of air acts downward
