This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Immunology, Subject name: Microbiology
[{'content': 'Membrane attack complex is formed by all except? \nSelect and explain. \nA. C3, B. C5, C. C7, D. C9', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. (a) C3', 'role': 'assistant'}]