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': 'Age was found to be a factor affecting the relation of diabetes mellitus and periodontics \nSelect and explain. \nA. Sampling Bias, B. Selection Bias, C. Confounding Bias, D. Information Bias', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]