[观察例句]
1．I guess you have been asked about the title of your book before.
2．New inventions like 3D printers have been used to make replacement hearts and bone parts.
3．I guess someone has asked you about the title of your book before.
4．People have used new inventions like 3D printers to make replacement hearts and bone parts.
5．Something bad has happened to him.
6．The door has opened of itself.
7．The orphan has been well looked after.
