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
[{'content': 'Floor of 4th ventricle is not formed by? \nSelect and explain. \nA. Posterior surface of pons, B. Sulcus limitans, C. Posterior surface of medulla, D. Inferior medullary velum', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Inferior medullary velum', 'role': 'assistant'}]