How the big forest fire has ________ remains to be a mystery.
Mrs. Black took the police back to ______ place ______ she witnessed the robbery.
______ her hard work, she succeeded in passing the exam.
Jenny pushed the door again to ______ it ______.
The teacher suggested _____ that we should make the most of time to study English well.
He said he would prefer ______ others rather than ______ by others.