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
Which of the following is not an alkylating agent ? 
Select and explain. 
A. 5–FU, B. Busulfan, C. Cyclophosphamide, D. Melphalan
A: 5–FU
Answer: A: 5–FU 
Explanation: None