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
True about 1gM: 
Select and explain. 
A. Is dimeric, B. Activates complement, C. Is not synthesized by fetus, D. Is the last to cross
B: Activates complement
Answer: B: Activates complement 
Explanation: Activates complement