–Our classroom twice a day. So it is very clean.–It is important for us to do this.A. is

问题补充：

–Our classroom twice a day. So it is very clean.

–It is important for us to do this.

A. is sweeping

B. is swept

C. sweeps

D. has swept

答案：

A. is sweeping

(答案→)B. is swept

C. sweeps

D. has swept

解析：句意：我们的教室一天打扫两次。所以它很干净。对我们来说打扫教室很重要。根据句意，可知句子要用被动形式，本题中说明的是一般的情况，故用一般现在时的被动，故选B