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 common site of obstruction in chronic dacryocystitis in adults is: \nSelect and explain. \nA. At upper end of nasolacrimal duct, B. At lower end of nasolacrimal duct, C. In the lacrimal sac, D. In the common canaliculus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. At upper end of nasolacrimal duct', 'role': 'assistant'}]