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': 'The size of periapical film used in adults is \nSelect and explain. \nA. 21 x 31 mm, B. 31 x 41 mm, C. 22 x 35 mm, D. None of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]