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
Subgingival calculus 
Select and explain. 
A. 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
C: is uncommon in children
Answer: C: is uncommon in children 
Explanation: None