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
Mean exposure of radiation from an intraoral periapical radiograph: 
Select and explain. 
A. 300 mR, B. 90 mR, C. 0.03 R, D. 10 mR
A: 300 mR
Answer: A: 300 mR 
Explanation: Mean exposure of radiation from an intraoral periapical radiograph is 300 mR.