Cyclin dependent kinases (Cdk) are proteins that work with cyclin regulatory proteins. The cyclins are associated with different checkpoints of the cell cycle. Different phases of the cell cycle experience activation and/or deactivation of specific cyclin-Cdk complexes. Referring to the graph, what phase is there a drastic decrease in three of the cyclins?

A) Mitosis
B) S phase
C) G1 phase
D) G2 phase