The three major checkpoints of the cell cycle
a. are located at the M/G1 boundary, the S/G2 boundary, and within Sphase
b. are located at the G1/5 boundary, the G2/M boundary, and within M phase
c. are located between G1 and Go, and at the S/G2 and M/G1 boundaries
d. are located at the prophase, anaphase, and telophase stages of mitosis