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
Ultrasonic devices are instruments for the removal of dentin from root canal walls have proved to be 
Select and explain. 
A. Effective, B. Disappointing, C. Lethal, D. None of the above
B: Disappointing
Answer: B: Disappointing 
Explanation: None