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: Surgery
[{'content': 'Malignant ulcer is differentiated from benign by \nSelect and explain. \nA. Heaping up of margins, B. Fibrous scars radiating from crater, C. Induration of base, D. clean base', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]