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': 'A distal shoe type of space maintainer is indicated in \nSelect and explain. \nA. Loss of primary anterior tooth, B. If the primary 2nd molar is lost before the eruption of permanent 1st molar, C. If the primary 2nd molar is lost after the eruption of permanent 1st molar, D. Loss of primary 1st molar', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]