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
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 
Select and explain. 
A. Cocci, B. Spirochetes, C. Bacteriophages, D. Filamentous rods
B: Spirochetes
Answer: B: Spirochetes 
Explanation: None