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
All are alkylating agents except : 
Select and explain. 
A. Cyclophosphamide, B. Lomustine, C. Busulfan, D. Zalcitabine
D: Zalcitabine
Answer: D: Zalcitabine 
Explanation: None