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
Parents should begin cleaning a child's mouth 
Select and explain. 
A. By the time first tooth erupts, B. When all deciduous teeth erupt, C. When all anterior teeth have erupt, D. When incisors and first molars have erupt
A: By the time first tooth erupts
Answer: A: By the time first tooth erupts 
Explanation: None