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: Pathology
Which of the following bacteria is initiator of smooth surface caries: 
Select and explain. 
A. Streptococcus mutans., B. Streptococcus sanguis., C. Lactobacillus., D. Actinomyces.
A: Streptococcus mutans.
Answer: A: Streptococcus mutans. 
Explanation: None