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: Pathology
Why fetal cells continue to divide but terminally differentiated adult cells do not divide? 
Select and explain. 
A. 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
A: There are many cyclin inhibitors which prevent cell to enter into S phase in adult
Answer: A: There are many cyclin inhibitors which prevent cell to enter into S phase in adult 
Explanation: None