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: Radiology
[{'content': 'A 15 year old male patient complains of pain in the lower back tooth. History of frequent snacking in between meals.\nIntra-oral examination reveals occlusal caries with 16 and 36, proximal caries between 26 and 27 and grossly carious 46.\nWhich of the following must be used while taking a radiograph for this patient? \nSelect and explain. \nA. Round collimator, B. Rectangular collimator, C. Short pointed aiming tubes, D. Short source-to-skin distance', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]