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
Kaposi's sarcoma is a tumour of: 
Select and explain. 
A. Blood vessels, B. Reticuloendothetial system, C. Striated muscles, D. Smooth muscles
A: Blood vessels
Answer: A: Blood vessels 
Explanation: None