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 operator should stand at a distance of _______ while
taking X-ray. 
Select and explain. 
A. 6 feet, B. 8 feet, C. 10 feet, D. 2 meters
A: 6 feet
Answer: A: 6 feet 
Explanation: Means for Reducing X-Ray Exposure: 

Use selection criteria to assist in determining type and frequency of radiographic examinations. 
Use E/F-speed film or digital sensors. 
Use holders to support film or digital sensors intraorally.
Make exposures with 60 to 70 kVp. 
Replace short pointed aiming tubes with open-ended aiming cylinders. 
Use rectangular collimation for periapical and bitewing images. 
Use thyroid collars. 
Stand at least 6 feet (2 m) away from patient and away from the X-ray machine (preferably out of X-ray room) when making exposure. 
With film, use time-temperature film processing rather than "sight" processing, or use an automatic processor. 
Use rare-earth screens for panoramic and cephalometric film imaging or use digital systems. 
Reduce cone-beam CT beam field of view to region of interest. 

Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 33