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': 'Subgingival calculus \nSelect and explain. \nA. cannot occur without supragingival calculus, B. contains less hydroxyapatite than supragingival calculus, C. is uncommon in children, D. derives its minerals from same source as supragingival calculus', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]