We must keep the classroom .A. cleanB. to cleanC. cleaningD. cleaned

问题补充：

We must keep the classroom .

A. clean

B. to clean

C. cleaning

D. cleaned

答案：

(答案→)A. clean

B. to clean

C. cleaning

D. cleaned

解析：本题的含义是我们必须要使教室保持干净，keep sth+形容词表示使某物保持某个状态，clean可以为形容词，故本题选A。