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: Pathology
Haemophilia B is due to mutation of gene corresponding to factor? 
Select and explain. 
A. Factor 8, B. Factor 9, C. Factor 7, D. Factor 11
B: Factor 9
Answer: B: Factor 9 
Explanation: None