The outer ear channels sound waves down the external __________ to the __________, a tight membrane that vibrates in response to sound waves.

Sound waves travel through the external auditory canal and then reach the eardrum, a thin membrane that vibrates in response to sound. The oval window is deeper inside the ear, and the pinna is the visible outer part that collects sound rather than the canal it travels through. So the correct choice is “auditory canal . . . eardrum.”

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