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': 'The periapical radiograph of RCT treated tooth shows wiggly apical seal of master cone is due to: \nSelect and explain. \nA. Apical perforation of master cone, B. Excess cement in apical region, C. Obstruction of master cone at apical third, D. Short master cone in apical third', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]