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: Microbiology
[{'content': 'Electron microscopic examination of the bacterial flora of necrotising ulcerative gingivitis indicates the presence of microorganisms within non-necrotic tissues in advance of other bacteria. The organisms involved are \nSelect and explain. \nA. Cocci, B. Spirochetes, C. Bacteriophages, D. Filamentous rods', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]