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 best investigation for colorectal carcinoma - \nSelect and explain. \nA. Exfoliative cytology, B. Air contrast barium enema, C. Ultrasound, D. Colonoscopy and biopsy', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]