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': 'Investigation of choice in suspected gallbladder stone is \nSelect and explain. \nA. Ultrasound, B. X-Ray, C. Barium study, D. Oral cholecystography', 'role': 'user'}, {'content': 'Answer: A \nExplanation: USG can demonstrate Biliary calculi Size of GB and CBD Thickness of GB wall Presence of inflammation around GB Occasionally presence of stones within the biliary tree Ref: Sabiston 20th edition Pgno :1487', 'role': 'assistant'}]