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
[{'content': "Which of the following anti–cancer drug is NOT 'S'–phase specific ? \nSelect and explain. \nA. Methotrexate, B. Mercaptopurine, C. Ifosfamide, D. Thioguanine", 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]