问题补充：
—The classroom is clean it was yesterday.
— Sorry, I forget to clean it.
A. as; as
B. so; as
C. not so; as
D. more; than
答案：
A. as; as
B. so; as
(答案→)C. not so; as
D. more; than
解析：as; as 和……一样；so; as和……一样，用于否定句；not so; as不如……；more; than比……更。句意：教室不如昨天那样干净。——抱歉，我忘了打扫。结合语境可知选C。
