This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nose and paranasal sinuses, Subject name: ENT
Which is not a pa of paranasal sinus? 
Select and explain. 
A. frontal, B. Ethmoid, C. Sphenoid, D. Pyriform
D: Pyriform
Answer: D: Pyriform 
Explanation: Paranasal sinuses are air containing cavities in ceain bones of skull. They are four on each side. Clinically,paranasal sinuses have bern divided into Anterior-Maxillary,Frontal, Anterior ethmoidal. posterir-Posterior ethmoid,sphenoid sinus. Ref Dhingra 6/e,p 187.