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: Surgery
[{'content': 'Which of the following have greatest concentration of osteogenic cells: \nSelect and explain. \nA. Marrow- cancellous graft., B. Marrow- cortical graft., C. Costochondral graft., D. Cortical graft', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]