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': 'Which of the following bacteria is initiator of smooth surface caries: \nSelect and explain. \nA. Streptococcus mutans., B. Streptococcus sanguis., C. Lactobacillus., D. Actinomyces.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]