【急!】将下列句子变为被动语态.

Stamps can be bought at the post office.
Ball games are not permitted in this area.
Bags should not be left by the passengers unattended.
All books must be returned to the library on time.
Sneakers are not allowed in school.

1.Stamps can be bought at the post office.
2.Ball games are not permitted in this area.
3.Bags should not be left by the passengers unattended.
4.All books must be returned to the library on time.
5.Sneakers are not allowed in school.
应该都对，我以前做过