How Is the Positive Pressure Level Established in a Bubble

How is the positive pressure level established in a bubble CPAP system?

A) The therapist dials the desired CPAP level directly on the ventilator.
B) The therapist immerses the distal end of the expiratory limb a certain distance below the water surface.
C) The CPAP level is established by stacking adaptors with weighted balls to the distal opening of the expiratory limb.
D) The positive pressure is achieved by tightening a screw clamp attached to the expiratory limb until the desired pressure is achieved.