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Inhalation is driven primarily by the diaphragm. When the diaphragm contracts, it flattens and moves downward, increasing the vertical size of the chest cavity. This expansion lowers the air pressure inside the lungs relative to the outside air, so air is drawn in.

The rib cage may also expand as intercostal muscles lift the ribs, aiding the process, but the essential action initiating inhalation is the diaphragm contracting and moving downward. The other statements describe exhalation or incorrect timing of muscle action (diaphragm relaxing or moving upward).