The is the main breathing muscle that expands the lungs when it contracts and forces air into the lungs. larynx trachea lungs diaphragm

The diaphragm is the right answer!

"Diaphragm. The diaphragm is the major muscle responsible for breathing. It is a thin, dome-shaped muscle that separates the abdominal cavity from the thoracic cavity. During inhalation, the diaphragm contracts, so that its centre moves caudally (downward) and its edges move rostrally (upward)."

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