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
During which phase of the cell cycle, the cellular content of DNA is doubled? 
Select and explain. 
A. Mitotic phase, B. G1 phase, C. G2 phase, D. S phase
D: S phase
Answer: D: S phase 
Explanation: The cellular content of DNA is doubled during the S phase.