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: Pharmacology
Drug not acting on tubulin is : 
Select and explain. 
A. Bleomycin, B. Colchicine, C. Paclitaxel, D. Vincristine
A: Bleomycin
Answer: A: Bleomycin 
Explanation: None