Compared to round collimator, use of rectangular collimator reduces the patient's skin surface exposed by:: (A) 15%, (B) 30%, (C) 45%, (D) 60%
Answer is D. 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