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。
