Cells experience distinct cycles of activities, such as growth and division. These activities are summarized in the image below. Beginning in the G1 phase, proteins called cyclins begin to accumulate in the cell. Cyclins combine with certain molecules to form cyclin-dependent kinase complexes (Cdkcs), which activate and deactivate other cellular proteins.
For example, cyclin B forms a Cdkc that controls the M stage of the cell cycle. This Cdkc turns on the proteins that are involved in mitosis, turns off the proteins that prevent mitosis, and even activates an enzyme complex that breaks down cyclin B. The loss of cyclin B signals the end of the M portion of the cycle. Notice that the concentrations of cyclin B peak during the mitosis stage.

Based on the above graph, which cyclin forms the primary Cdkc that controls the G2 stage?
A. Cyclin A
B. Cyclin E
C. Cyclin D
D. Cyclin B