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
All are secondary colonizers except 
Select and explain. 
A. S. sanguis, B. P. intermedia, C. Fusobacteria, D. P. gingivalis
A: S. sanguis
Answer: A: S. sanguis 
Explanation: None