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: Medicine
[{'content': 'Rheumatoid factor is mainly- \nSelect and explain. \nA. Ig M, B. Ig A, C. IgG, D. Ig D', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]