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': 'Best investigative modality for gall bladder - \nSelect and explain. \nA. OCG, B. PTC, C. Ultrasound, D. Intravenous cholangiogram', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]