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
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: 
Select and explain. 
A. ½ second, B. 1.5 second, C. 1 second, D. 2 seconds
D: 2 seconds
Answer: D: 2 seconds 
Explanation: None