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
Compared to round collimator, use of rectangular collimator reduces the patient's skin surface exposed by: 
Select and explain. 
A. 15%, B. 30%, C. 45%, D. 60%
D: 60%
Answer: D: 60% 
Explanation: 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. 
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 34