During breathing in humans, the contraction of external intercostal muscles causes

Explanation

The contraction of the external intercostal muscles during inhalation causes the ribs to move outward and upward. This action increases the volume of the thoracic cavity, allowing the lungs to expand and fill with air. Option B is correct because it describes this outward movement of the ribs. Option A is incorrect because compression of the rib cage would reduce lung volume, which is not the intended effect during inhalation. Option C is incorrect because the sternum moves outward, not inward, as the ribs move outward. Option D is incorrect because during inhalation, the diaphragm actually contracts and moves downward, which is the opposite of relaxation.