Technically, none of these answers are correct, but A is the best answer. CDKs are present, more or less, at a constant level no matter the cell cycle; it's the cyclins that, well, cycle, depending on which phase the cell is in. The simple interpretation is that each cycle has a "master" cyclin:CDK complex that's calling all the shots, and the fall of one specific cyclin:CDK pair and rise of a different pair ushers in a new phase of the cycle. The only cyclin thats regulated extrecellularly is G1/S (cyclin D); all others are on "automatic pilot" provided the conditions are met to proceed to the next phase.
