The question______new measures should be taken to control the terrible air pollution is______puzzles scientists a lot.
They often _______their friends at weekends and they always have a good time.
Do you think everything _______was true?
It was Tom and his friends that________for the serious accidents.
He was reading a very interesting story, completely_______himself in the story.
Whoever invented _______wheel is believed to have made ______most useful invention in _______history.