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
The following are alkylating agents except : 
Select and explain. 
A. Cyclophosphamide, B. Methotrexate, C. Mechlorethamine, D. Busulfan
B: Methotrexate
Answer: B: Methotrexate 
Explanation: None