参考答案：C

解析：固定搭配。根据原文“If he should fail to come in time，ask Bob to work in his place．”，此句是虚拟条件句，译文为：如果他不能按时到的话，让鲍勃代替他。本题关键在于理解固定搭配fail to do sth．，意思是：未能做成……，所以c选项if he didn’t come in time是对原文的正确解释。