The doctors are doing all that they _____, but the old man is still not breathing properly.

You __________ buy a gift, but you can if you want to.

According to the school rules, if a student _________ happen to damage something by accident,he/she should report it to a teacher or the office immediately.

The room is in a terrible mess; it        have been cleaned.

You ___________ finish the report today. Any time before this weekend is OK.