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 do 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: . There are many cyclin inhibitors which prevent cell to enter into S phase in adult