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
The ultrasonic scaler dislodges only calculus that 
Select and explain. 
A. has been formed recently, B. is already loose, C. is in direct contact with the tip, D. is located on the interproximal surfaces
C: is in direct contact with the tip
Answer: C: is in direct contact with the tip 
Explanation: None