Sheldon could have defeated Howard in the match but he too careless at the last minute.A

问题补充：

Sheldon could have defeated Howard in the match, but he too careless at the last minute.

A. is

B. was

C. has been

D. had been

答案：

A. is

(答案→)B. was

C. has been

D. had been

解析： 句意：Sheldon本可以在比赛中打败 Howard的，但是他最后一分钟太粗心了。前面的could have defeated是对过去情况的虚拟，but后面的内容是过去的事实，用一般过去式，所以选B。