问题补充:
Bob could have seen the famous pop star yesterday morning.
A. did he come
B. if he came
C. had he come
D. if he should come
答案:
A. did he come
B. if he came
(答案→)C. had he come
D. if he should come
解析:考查虚拟语气和倒装。在条件句中,表示与过去事实相反的虚拟语气时,条件句中使用过去完成时的时态。如果省略了if可以把had提前形成部分倒装的形式。本题的had he come yesterday morning=if he had come yesterday morning。