This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Radiology
[{'content': 'Investigation of choice for choledocolithiasis is \nSelect and explain. \nA. CT, B. USG, C. PET scan, D. HIDA scan', 'role': 'user'}, {'content': 'Answer: A \nExplanation: (Ref: Grainger and Allison&;s diagnostic radiology 6th edition, chapter 32)', 'role': 'assistant'}]