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': 'True about 1gM: \nSelect and explain. \nA. Is dimeric, B. Activates complement, C. Is not synthesized by fetus, D. Is the last to cross', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Activates complement', 'role': 'assistant'}]