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 most probable cause of crowding in lower anterior region is: \nSelect and explain. \nA. Prolonged retention of lower primary incisors, B. Premature exfoliation of tower primary incisors, C. Presence of supernumerary teeth, D. Tooth-size-arch length discrepancy', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]