This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Operative Surgery, Subject name: ENT
In FESS surgery structure preserved is? 
Select and explain. 
A. Ethmoid sinus, B. Sphenoid sinus, C. Maxillary sinus, D. None
C: Maxillary sinus
Answer: C: Maxillary sinus 
Explanation: ANSWER: (C) Maxillary sinusREF: Textbook Of Oral And Maxillofacial Surgery (full Colour) 2nd edition by Neelima Anil Malik Page 583TYPES OF FUNCTIONAL ENDOSCOPIC SINUS SURGERY:Type 1Nasal endoscopy and uncinectomy with or without agger nasi cell exenteration.Type 2Nasal endoscopy, uncinectomy, bulla ethmoidectomy, removal of sinus lateralis mucous membrane and exposure of frontal recess/ frontal sinusType 3Type 2 plus maxillary sinus antrostomy through the natural sinus ostiumType 4Type 3 plus complete posterior ethmoidectomyType 5Type 4 plus sphenoidectomy & stripping of mucous membrane