The Sphenoid Sinus Is

The sphenoid sinus is:

A) cavity in the skull which opens into the nasal cavity located in the center of the skull base
B) small flap of cartilage that closes over the trachea preventing food from entering the larynx and thus the lungs
C) cavity in the skull which opens into the nasal cavity located between the nose and eyes
D) tubular structure which branches off the trachea