Answer:
B. Concentrations of proteins in the cytoplasm reach the appropriate level
Explanation:
We can say that the cell cycle checkpoints is an important cell cycle control. In this regard, it is important to note that within a cell, the passage from one phase to another is controlled by regulatory factors that are characteristically proteinic and act at the so-called cell cycle checkpoints.
A cell passes the first checkpoint when protein concentrations in the cytoplasm reach the appropriate level.
