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
Pregnancy gingivitis is caused by: 
Select and explain. 
A. Bacteriodes melaninogenicus, B. Actinobacillus actinomycetemcomitans, C. Streptococcus sanguis, D. Fusobacterium
A: Bacteriodes melaninogenicus
Answer: A: Bacteriodes melaninogenicus 
Explanation: None