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 anti–cancer drug is NOT 'S'–phase specific ? 
Select and explain. 
A. Methotrexate, B. Mercaptopurine, C. Ifosfamide, D. Thioguanine
C: Ifosfamide
Answer: C: Ifosfamide 
Explanation: None