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
Pulmonary fibrosis is caused by : 
Select and explain. 
A. Methotrexate, B. Cyclophosphamide, C. Mercaptopurine, D. Busulfan
D: Busulfan
Answer: D: Busulfan 
Explanation: None