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
Which stage is characterized by beginning of mineralization and root formation: 
Select and explain. 
A. Advanced Bell Stage., B. Early Bell Stage., C. Bus Stage ., D. Cap Stage.
A: Advanced Bell Stage.
Answer: A: Advanced Bell Stage. 
Explanation: None