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
A step-wedge phantom is used to assess the quality
assurance of: 
Select and explain. 
A. Dark room, B. Timer of the X-ray machine, C. Chemicals used for processing, D. Radiation leakage from X-ray unit
C: Chemicals used for processing
Answer: C: Chemicals used for processing 
Explanation: How frequently processing solutions are replaced depends primarily on the rate of use of the solutions but also on the size of tanks, whether a cover is used, and the temperature of the solutions. In most offices, the solutions should be changed weekly or every other week. The results of the step-wedge test help determine the proper frequency.
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 251