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': 'In Which year were powered toothbrushes invented? \nSelect and explain. \nA. 1921, B. 1929, C. 1939, D. 1941', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]