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': 'Length of the nasolacrimal duct is about: \nSelect and explain. \nA. 8-12 mm, B. 22-34 mm, C. 12-18 mm, D. 18-21 mm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Ans. 12-18 mm', 'role': 'assistant'}]