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': 'What must an instrument do to completely clean the canal space? \nSelect and explain. \nA. Be deflected at the canal orifice, B. Be 2 to 3 mm short of the radiographic apex, C. Fit loosely into the canal, D. Contact all walls and surfaces', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]