几句被动语态 请把原句改成被动语态

I was asked to speak at the meetingThe car was seem hit into the postHe is considered (by us) to be the best singerThese questions had to be thought over (by them)This opportunity should be taken advantage of (by us)Bicycles mustn't be left in the hallThis couldn't have been done by himThe cup was sure to be won by himPollution 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