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: Ophthalmology
[{'content': 'Most vascular structure of eyeball is \nSelect and explain. \nA. Iris, B. Ciliary body, C. Choroid, D. Sclera', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]