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': 'Mineral trioxide aggregate used in apexification: \nSelect and explain. \nA. Forms an integral part of the root canal filling, B. Dissolves as the apical barrier formation progresses, C. Has not yielded good results, D. Is available in a paste form', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]