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
[{'content': 'A step-wedge phantom is used to assess the quality\nassurance of: \nSelect and explain. \nA. Dark room, B. Timer of the X-ray machine, C. Chemicals used for processing, D. Radiation leakage from X-ray unit', 'role': 'user'}, {'content': "Answer: C \nExplanation: 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.\nReference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 251", 'role': 'assistant'}]