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': 'Iris is thinnest at: \nSelect and explain. \nA. Ciliary zone, B. pupil margin, C. Base, D. Collarette', 'role': 'user'}, {'content': 'Answer: C \nExplanation: The iris is thinnest at its attachment to the ciliary body (base) so that if it tends to give way in this region.', 'role': 'assistant'}]