Incomplete expansion of a lung or portion of a lung is called:

A. Quiet inspiration involves the diaphragm and external intercostal muscles without significant involvement of the neck and abdominal muscles.
B. Forceful exhalation (such as during coughing or strenuous activities) engages the abdominal muscles and sometimes the neck muscles (such as the sternocleidomastoid), as they help to push air out of the lungs by increasing intra-abdominal and intrathoracic pressure.
C. None of the answers are correct is incorrect because forceful exhalation involves these muscles.
D. Quiet exhalation primarily relies on passive recoil of the lungs and relaxation of the diaphragm, with minimal involvement of the neck or abdominal muscles.

A. Bicarbonate is a buffer in the blood that helps maintain pH balance, but it is not a gas and does not directly regulate breathing.
B. Carbon dioxide (CO₂) is the primary regulator of respiration. Elevated CO₂ levels in the blood lower pH (making it more acidic), which stimulates chemoreceptors to increase the rate and depth of breathing to expel more CO₂ and restore balance.
C. Nitrogen makes up the majority of the air we breathe, but it is inert and plays no active role in regulating respiration.
D. Oxygen does influence breathing, especially at very low levels, but it is not the primary driver. In healthy individuals, CO₂ levels are the main factor controlling respiratory rate.