Final answer:
An Oxygen cylinder is not a component of an open-circuit breathing apparatus; it is used in closed-circuit systems. Open-circuit scuba gear is comprised of a regulator, a face piece, and a low-pressure alarm, and uses compressed air.
Step-by-step explanation:
The component that is not a part of an open-circuit breathing apparatus is a D. Oxygen cylinder. Open-circuit systems are commonly used by scuba divers and they use compressed air, not pure oxygen. The elements that are typically included in an open-circuit scuba apparatus are a regulator to control the flow of air, a face piece which could be a mask or mouthpiece to deliver the air to the diver, and a low-pressure alarm to warn of the air supply getting low. An oxygen cylinder would typically be found in a closed-circuit rebreather system which is used for technical diving or situations where the bubble release needs to be minimized.
The partial pressure of oxygen is lower in the lung than in the external air due to several factors including humidification of the air in the lungs, mixing of the incoming air with residual air in the lungs which already has a lower oxygen content, and the constant uptake of oxygen into the bloodstream. This is not directly related to the apparatus but is a factor to consider when understanding respiration under different circumstances, like underwater.