把下列句子改为被动语态.1、They have built many tall buildings .
Many tall buildings ____ ____ ____ .
2、You must hand in your history homework by the weekend .
Your history homework ____ ____ ____ ____ by the weekend .
3、Last Sunday we heard them sing Beijing Opera in the theater .
They ____ ____ ____ ____ Beijing Opera in the theater last Sunday .
4、You should water the flowers once a day .
The flowers ____ ____ _____ once a day .
5、Do people grow rice in North China
____ rice ____ in North China
6、Didn't they tell you about it
____ you ____ about it