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: Surgery
All of the following are known predesposing factors for cholangiocarcinoma except - 
Select and explain. 
A. CBD stones, B. Clonorchis sinensis, C. Ulcerative colitis, D. Primary sclerosing cholangitis
A: CBD stones
Answer: A: CBD stones 
Explanation: None