This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anatomy, Subject name: Ophthalmology
[{'content': 'Size of optic disc is? \nSelect and explain. \nA. 0.5 mm, B. 1.5 mm, C. 5.5 mm, D. 10.5 mm', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Ans. (b) 1.5 mmRef.: Kanski 7thed chapter 10, Glaucoma: Science and Practice By John Morrison, Irvin Pollack p. 91', 'role': 'assistant'}]