Final answer:
a) Cyclins appear in some stages of the cell cycle and are degraded at other stages.
Step-by-step explanation:
Cyclins appear in some stages of the cell cycle and are degraded at other stages. They are a class of proteins that regulate the progression of the cell cycle by interacting with cyclin-dependent kinases (Cdks). The concentration of cyclin proteins fluctuates throughout the cell cycle, increasing in the previous stage and then being degraded after the cell moves to the next stage.