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
A 9-year-old patient exhibits left maxillary central incisor in cross bite. Supporting bone is in harmony with tooth size. The most probable cause is: 
Select and explain. 
A. Premature extraction of primary right central incisor, B. Prolonged retention of primary Left central incisor, C. Absence of mandibular left central incisor, D. All of the above
B: Prolonged retention of primary Left central incisor
Answer: B: Prolonged retention of primary Left central incisor 
Explanation: None