几句被动语态 请把原句改成被动语态

I was asked to speak at the meeting
The car was seem hit into the post
He is considered (by us) to be the best singer
These questions had to be thought over (by them)
This opportunity should be taken advantage of (by us)
Bicycles mustn't be left in the hall
This couldn't have been done by him
The cup was sure to be won by him
Pollution must be get rid of (by us)
The problem cannot be worked out by anyone
括号中的是可写可不写的