——Why is the classroom so dirty?
——Sorry, Sir. It yesterday. We forgot to do it.
A. doesn't clean
B. didn't clean
C. isn't cleaned
D. wasn't cleaned
网友回答
A. doesn't clean
B. didn't clean
C. isn't cleaned
(答案→)D. wasn't 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。