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': 'Which is true of Ulcerative colitis \nSelect and explain. \nA. String sign of kantor positive, B. Skip lesions are seen, C. Rectum is always involved, D. Fistulas are common', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]