Work Step by Step
B. is correct because once the cell has begun to synthesize another copy of its DNA (entered S phase), it cannot stop without causing a genetic disaster. C. is also right since, once the sister chromatids separate at the start of anaphase, there is not way to reattach them. In contrast, A. is wrong because the cell could go back to S phase to finish copying DNA, making the step reversible. As B. and C. are both correct, D. is the answer.