For an average human fibroblast, the time taken for a cell to divide into two daughter cells is between 18 and 24 hours. In a laboratory experiment, human fibroblast cells are exposed to growth factors for one hour, and then a compound that inhibits cyclin-dependent kinases (CDK) is added to the cells. Which of the following statements best describes the state of fibroblast cells after addition of the CDK inhibitor?

A) The fibroblasts fail to go through the G1 checkpoint and remain in the G1 phase.
B) The fibroblasts fail to go through the G2 checkpoint and remain in the G2 phase.
C) The fibroblasts have passed the G1 checkpoint and have entered the S phase, where they remain.
D) The fibroblasts have passed the G1 and G2 checkpoints and have entered the M phase, where they remain.