This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: G.I.T, Subject name: Anatomy
[{'content': 'Best investigation modality for gallbladder \nSelect and explain. \nA. OCG, B. PTC, C. Ultrasound, D. Intravenous cholangiogram', 'role': 'user'}, {'content': 'Answer: C \nExplanation: USG is the investigation of choice for Calculous cholecystitis Chronic cholecystitis Cholelithiasis Ref: Sabiston 20th edition Pgno : 1487', 'role': 'assistant'}]