This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neoplastic disorders of WBCs, Subject name: Pediatrics
[{'content': 'Most common type of ALL in children: \nSelect and explain. \nA. Pre-B cell ALL, B. Mature B cell ALL, C. Pre-T cell ALL, D. Mature T cell ALL', 'role': 'user'}, {'content': 'Answer: A \nExplanation: - 85% cases of ALL are pre-B-ALL - 15% are T-ALL and 1% are derived from mature B cells', 'role': 'assistant'}]