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
Rheumatoid factor in rheumatoid arthritis is important because -a) RA factor is associated with bad prognosisb) Absent RA factor rules out the diagnosis of Rheumatoid arthritisc) It is very common in childhood Rheumatoid arthritisd) It correlates with disease activity 
Select and explain. 
A. b, B. ad, C. ac, D. ab
B: ad
Answer: B: ad 
Explanation: None