This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
During extraction of the upper first molar, the mesio buccal root is missing and is suspected to have been pushed into the maxillary sinus.
To close a oro antral fistula which of the flaps cannot be used 
Select and explain. 
A. Buccal Flap, B. Palatal Flap, C. Tongue flap, D. Mucus Flap
D: Mucus Flap
Answer: D: Mucus Flap 
Explanation: None