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: Anatomy
Floor of 4th ventricle is not formed by? 
Select and explain. 
A. Posterior surface of pons, B. Sulcus limitans, C. Posterior surface of medulla, D. Inferior medullary velum
D: Inferior medullary velum
Answer: D: Inferior medullary velum 
Explanation: Inferior medullary velum