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
The dental x-ray beam is collimated to a circle of: 
Select and explain. 
A. 7 cm in diameter, B. 5 cm in diameter, C. 6 cm in diameter, D. 9 cm in diameter
A: 7 cm in diameter
Answer: A: 7 cm in diameter 
Explanation: Collimation is a process by which x-ray beam size is reduced and thereby decrease the patient exposure. 
It is made up of metallic barrier usually Pb with an aperture in the middle to reduce the size of the x-ray beam.
Most state regulations require that the x-ray beam used in intraoral radiography  be  collimated  so  that  the  field  of  radiation  at  the patient’s skin surface is no more than 7 cm (2.75 inches) in diameter.