Answer: Yes definitely Cyclin A
Explanation: The concentrations of cyclin A peak during the G2 stage, suggesting that the start of this stage is triggered by high concentrations of the Cdkcs it forms. Similarly, cyclin A vanishes around the end of the G2 stage. These two facts together indicate that the G2 cycle is controlled by Cdkcs formed with cyclin A.
Similarly, cyclin E controls the transition from G1 to S.
These examples show how complex interactions among the different kinds of molecules in the cell cause distinct cycles of activities.