Answer
A.
Work Step by Step
Cdk's must bind to cyclins in order to be active, and the regular appearance and disappearance of cyclins in turn regulates the Cdk's (A.). B. and C. are events in the cell cycle which are themselves regulated by the action of cyclins and Cdk's, not the other way around, making these choices incorrect, as well as D. which requires B. to be correct.