This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
[{'content': 'Why fetal cells continue to divide but terminally differentiated adult cells do not divide - \nSelect and explain. \nA. There are many cyclin inhibitors which prevent cell to enter into S phase in adult, B. Phosphatase absent in fetal cells, C. Proteinase is absent in fetus, D. Absence of CD kinase', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A cyclin-dependent kinase inhibitor protein is a protein which inhibits cyclin-dependent kinase. Several function as tumor suppressor genes. Cell cycle progression is negatively controlled by cyclin-dependent kinases inhibitors (called CDIs, CKIs or CDKIs). CDIs are involved in cell cycle arrest at the G1 phase.', 'role': 'assistant'}]