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': 'pH for acidifier used in fixer should be: \nSelect and explain. \nA. 3.2 - 4.2, B. 4 - 4.5, C. 4.5 - 5, D. 5.2 - 5.5', 'role': 'user'}, {'content': "Answer: B \nExplanation: The fixing solution contains an acetic acid buffer system (pH 4 to 4.5) to keep the fixer pH constant. The acidic pH is required to promote good diffusion of thiosulfate into the emulsion and of silver thiosulfate complex out of the emulsion. The acid-fixing solution also inactivates any residual developing agents in the film emulsion, blocking continued development of any unexposed crystals while the film is in the fixing tank.\nWhite and Pharoah's Oral\xa0 Radiology Principles and Interpretation 8th edition", 'role': 'assistant'}]