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': 'The most useful investigation for profuse lower gastrointestinal bleeding is- \nSelect and explain. \nA. Proctosigmoidoscopy, B. Colonoscopy, C. Double contrast barium enema, D. Selective arteriography', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]