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: Social & Preventive Medicine
In the given data, degree of freedom will be -
 Duration of developing AIDSBlood Group AB C D 0-10 years20 30 48 710-20 years 100 15 38 10 20-30 years 12 9 8 3 
Select and explain. 
A. 12, B. 6, C. 9, D. 20
B: 6
Answer: B: 6 
Explanation: None