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: Social & Preventive Medicine
[{'content': 'Basic types of biases in epidemiological study-a)  Investigation biasb)  Subject biasc)  Random errord)  Analyzer bias \nSelect and explain. \nA. dc, B. cb, C. abd, D. b', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]