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: Forensic Medicine
[{'content': 'Which of the following methods is used for demonstrating old washed up blood-stains - \nSelect and explain. \nA. Infrared photography, B. Luminol spray, C. Ultraviolet light, D. Magnifying lens', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Luminol (3-aminophthalhydrazide) spray is most common blood reagent used for visualizing/demonstrating weak/ faint blood stains i.e old washed up or cleaned up stains and blood stains in areas difficult to see such as outdoor vegetations etc.', 'role': 'assistant'}]