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
Transition from G2 to M phase of the cell cycle is controlled by: 
Select and explain. 
A. Retinoblastoma gene product, B. p53 protein, C. Cyclin E, D. Cyclin B
D: Cyclin B
Answer: D: Cyclin B 
Explanation: None