问题补充:
——Why is the classroom so dirty?
——Sorry, Sir. It yesterday. We forgot to do it.
A. doesn clean
B. didn clean
C. isn cleaned
D. wasn cleaned
答案:
A. doesn clean
B. didn clean
C. isn cleaned
(答案→)D. wasn cleaned
解析:本句子考察的是一般过去时的被动态,其结构主语+was/were+动词过去分词,根据中的yesterday,可推知。