将下一句子改为被动语态（九年级英语）,能改几句就改几句,高手来
1.Who bought the book?
2.We saw him sitting there without doing anything
3.I will ring Tom up
4.Someone has taken the stranger to another hospital
5.Did he do his homework last night?
6.The students listened to the teacher carefully
7.He asked to play the piano last Sunday