This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
[{'content': 'Most virulent dengue fever strain is - \nSelect and explain. \nA. 1, B. 2, C. 3, D. 4', 'role': 'user'}, {'content': 'Answer: B \nExplanation: 4 types of dengue virus exist. DEN1,DEN2,DEN3,DEN4,in which 2 is more virulent REF:ANANTHAN ARIYANAM of your TEXT BOOK OF MICROBIOLOGY 9EDITION PGNO.523', 'role': 'assistant'}]