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
[{'content': 'How long before obturation should the calcium hydroxide paste be allowed to remain in canal, if the apexiﬁcation of open apex is intended? \nSelect and explain. \nA. 2 weeks, B. 4 weeks, C. 3 months, D. 6 months', 'role': 'user'}, {'content': 'Answer: D \nExplanation: As a general rule, the treatment paste is allowed to remain for 6 months. The root canal is then\xa0reopened to determine whether the tooth is ready for a conventional gutta-percha ﬁlling, as determined by the presence of a “positive stop” when the apical area is probed with a ﬁle. Often, there is also radiographic evidence of apical closure.\xa0\nFrank has described four successful results of\xa0apexiﬁcation treatment:\n\nContinued closure of the canal and apex to a normal appearance.\nA dome-shaped apical closure with the canal retaining a blunderbuss appearance.\nNo apparent radiographic change, but a positive stop in the apical area.\nA positive stop and radiographic evidence of a barrier coronal to the anatomic apex of the tooth.', 'role': 'assistant'}]