This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Brain , Subject name: Anatomy
[{'content': 'All of the following form the floor of third ventricle, except \nSelect and explain. \nA. Infundibulum, B. Occulomotor nerve, C. Mammillary body, D. Optic chiasma', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Floor of third ventricle is formed from before backward by :Optic chiasmaTuber cinerum & infundibulumMammillary bodiesPosterior perforated substanceTegmentum of midbrain(Ref: Vishram Singh textbook of clinical neuroanatomy second edition pg 137)', 'role': 'assistant'}]