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 collimation, rectangular collimation ↓es exposure by: 
Select and explain. 
A. 60%, B. 50%, C. 40%, D. 30%
A: 60%
Answer: A: 60% 
Explanation: Compared to round collimation, rectangular collimation ↓es exposure by 60%