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': 'At 90 kVp and 15 mA at a source film distance of 8 inches, the exposure time for a film is ½ second, using the same amount of kVp and mA, the exposure time at 16 inches is: \nSelect and explain. \nA. ½ second, B. 1.5 second, C. 1 second, D. 2 seconds', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]