Progression Through the Cell Cycle Requires a Cyclin to Bind

Progression through the cell cycle requires a cyclin to bind to a Cdk because

A) the cyclins are the molecules with the enzymatic activity in the complex.
B) the binding of a cyclin to Cdk is required for Cdk enzymatic activity.
C) cyclin binding inhibits Cdk activity until the appropriate time in the cell cycle.
D) without cyclin binding, a cell-cycle checkpoint will be activated.