This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Blood, Subject name: Medicine
Bleeding time is prolonged in: 
Select and explain. 
A. Von willibrand's disease, B. Christmas disease, C. Haemophilia, D. Polycythemia
A: Von willibrand's disease
Answer: A: Von willibrand's disease 
Explanation: Ans: a