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 ultrasonic scaler dislodges only calculus that \nSelect and explain. \nA. has been formed recently, B. is already loose, C. is in direct contact with the tip, D. is located on the interproximal surfaces', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]