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': 'Function of Ig A is - \nSelect and explain. \nA. Acts as a mucosal barrier for infection, B. Circulating antibody, C. Kills virus infected cells, D. Activates macrophages', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Question repeated', 'role': 'assistant'}]