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': "Compared to round PID, use of rectangular PID reduces the patient's skin surface exposed by: \nSelect and explain. \nA. 15%, B. 30%, C. 45%, D. 60%", 'role': 'user'}, {'content': "Answer: D \nExplanation: There are several means to limit the size of the x-ray beam. First, a rectangular position-indicating device (PID) may be attached to the radiographic tube housing. Use of a rectangular PID having an exit opening of 3.5 cm × 4.4 cm (1.38 inches × 1.34 inches) reduces the area of the patient’s skin surface exposed by 60% over that of a round (7 cm) PID.\nWhite and Pharoah's Oral\xa0 Radiology Principles and Interpretation 8th edition", 'role': 'assistant'}]