The door ________ open, no matter how hard she pushed.
It was ten o’clock _____ he came back home.
“Where _______ go to work?” “Where _______ go to work is not known.”
It was when she was about to go to bed ______ the telephone rang.
None knows if _______ that boy, but if _______ him, her parents will be disappointed.
The problem is _____ has caused us a lot of trouble.