Intercostal Muscle Contraction During Breathing Steps:

• Intercostal muscles (external) lie between ribs and contract to lift ribs upward and outward.
• This elevates the rib cage, expanding the thoracic cavity volume.
• Increased volume lowers air pressure inside the lungs compared to outside.
• Lower pressure draws air in, inflating lungs (Boyle's Law: pressure inversely proportional to volume).

Why B is correct:

• Contraction increases thoracic volume, decreasing intrapulmonary pressure per Boyle's Law, facilitating inhalation.

Why the others are wrong:

• A: Lungs inflate as a result of decreased pressure, not directly from muscle contraction.
• C: External intercostals raise ribs; downward movement occurs with internal intercostals during exhalation.
• D: Diaphragm movement is controlled by its own contraction, not intercostals.

Final answer: B