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
[{'content': 'At birth mouth is sterile,first bacteria to appear in oral cavity: \nSelect and explain. \nA. Streptococcus salivarius., B. Streptococcus mutans., C. Streptococcus sanguis., D. Streptococcus mitis.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]