问题补充：
---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
答案：
A. doesn’t clean
B. didn’t clean
C. isn’t cleaned
(答案→)D. wasn’t cleaned
解析：根据yesterday，可知本题考查的是一般过去时的被动语态，故选D
