句型转换：1、We clean our classroom twice a day .(改为被动语态）
Our classroom ____ ____ twice a day .
2、The teacher gave me a book .(改为被动语态)
I ___ ____ a book by the teacher .
3、Is the song sung by Sun Nan (改为主动语态）
____ Sun Nan ____ the song
4、The boss made the workers work 12 hours a day .(改为被动语态）
The workers ____ ____ ____ ____ 12 hours a day by the boss .