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: Dental
[{'content': 'Working length is \nSelect and explain. \nA. 0.5 — 1mm short of radiographic apex, B. 1-2 mm short of radiographic apex, C. Exactly at apex, D. 1-2mm apical to apex', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]