Final answer:
Once the cell passes through the restriction point (G1 checkpoint) and enters the S phase, it cannot return to G0 or G1.
Step-by-step explanation:
Once the cell passes through the restriction point (G1 checkpoint) and enters the S phase, it cannot return to G0 or G1. The G0 phase is a quiescent stage where cells are not actively preparing to divide. Cells in G0 may either resume the cell cycle and progress into G1 when conditions improve or receive an external signal triggering the onset of G1. However, once the cell enters the S phase, it is committed to completing the cell division process.