Fly B, actually the air flow from the airconditioning packs is relatively constant. The pressurization is controlled by a computer using the "outflow valve." The outflow valve is the largest opening in the fuselage once the doors are closed. During regulation mode, it is barely open. It opens just a slit and the opening is varied by the computer to maintain cabin altitude. It goes to full open on landing so in case passengers need to make an emergency exit you can open the doors and they won't be pressurized closed.
