This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General Microbiology Pa 1 (History, Microscopy, Stains and Structure and Physiology of Bacteria), Subject name: Microbiology
[{'content': 'Stain used in electron microscopy \nSelect and explain. \nA. 2.5% FAM, B. Phosphotungstic acid, C. Saffranin, D. Coomassie blue', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Phosphotungstic acid is electron dense, opaque for electrons. It is a common negative stain for viruses, nerves, polysaccharides and other biological tissue materials for imaging by an Electron microscope.', 'role': 'assistant'}]