This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Radiotherapy, Subject name: Radiology
The most radiosensitive cells are 
Select and explain. 
A. Bone-marrow cells, B. Muscle cells, C. Nerve cells, D. Breast glandular cells
A: Bone-marrow cells
Answer: A: Bone-marrow cells 
Explanation: The most sensitive cells are those that are undifferentiated, well nourished, dividing quickly and highly active metabolically. Amongst the body cells, the most sensitive are spermatogonia and erythroblasts, epidermal stem cells, gastrointestinal stem cells. The least sensitive are nerve cells and muscle fibres.Very sensitive cells are also oocytes and lymphocytes, although they are resting cells and do not meet the criteria described above. The reasons for their sensitivity are not clear.Reference: Eric J Hall Textbook of Radiobiology for Radiologist; 5th edition