答案：C

题目分析：句意：--对不起，史密斯教授，我昨天没有完成作业。--哦，你应该早完成它，因为昨天是最后期限。should have done指过去应该做的还未做，故选C。

点评：本题难度适中。

即学即练：I didn't know his telephone number， otherwise I ______ him.

A. had telephoned           B. would telephone    

C. would have telephoned     D. telephone

解析：C。句意：我不知道他的电话号码，要不然的话，我就给他打电话了。