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: Dental
Which one of the following complexes of periodontal micro-organisms is associated with bleeding on probing? 
Select and explain. 
A. Red Complex, B. Orange Complex, C. Green Complex, D. Yellow Complex
A: Red Complex
Answer: A: Red Complex 
Explanation: None