請將下列各句改寫成被動語態(tài).
1.People recognize Zheng Wenguang as the father of chinese science fiction.
2.We asked the teacher to explain the sentence again.
3.They are building a new house behind ourschool.
4.They were holding a meeting last night to discuss the problem.
5.The farmer has cut down the tall tree in his garden.
6.They had completed all the work when I arrived.
7.you can divide the books into three griups.