The correct option is A: one can consciously breathe in and breathe out by moving the diaphragm alone, without moving the ribs at all.
The diaphragm is the main muscle of inspiration and is under a considerable degree of voluntary control.
On conscious contraction, the diaphragm flattens and moves downward, increasing the vertical dimension of the thoracic cavity and drawing air into the lungs.
On relaxation, the elastic recoil of the lungs and chest wall expels air, so expiration can occur without active movement of the ribs.
With practice, most people can perform this diaphragmatic breathing while keeping the rib cage relatively still.
Option B is incorrect because the lungs cannot be made completely empty; a residual volume of air always remains in them.
Option C is incorrect because exhaled air is not oxygen-free; it still contains a considerable amount of unused oxygen.
Option D is incorrect because the Eustachian tube connects the middle ear with the nasopharynx and is involved in pressure equalisation, not in expelling air out of the body during breathing.
