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': 'Excessive fixing of an x-ray film: \nSelect and explain. \nA. Cause brown stains, B. Reduces the density, C. Increases the density, D. No effect on density', 'role': 'user'}, {'content': "Answer: B \nExplanation: The degree of darkening or opacity of an exposed film is referred to as optical density.\nExcessive fixation causes the image to become light, thereby reducing the optical density of the X-ray film.\nWhite and Pharoah's Oral\xa0Radiology Principles and Interpretation 8th edition", 'role': 'assistant'}]