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': 'CALCULUS all are true except: \nSelect and explain. \nA. Supragingival calculus is brown in colour, B. Hard and gritty, C. Calcified from plaque, D. Subgingival calculus is more difficult to remove than supragingival calculus', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]