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: Microbiology
[{'content': 'Which of the following is a defective virus - \nSelect and explain. \nA. HAV, B. HBV, C. HCV, D. HDV', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]