The maths problem remained confusing to the boy although _______ over and over.
Professor Williams keeps telling his students that the future ______to the well-educated.
It is______ of you to cooperate with such a bad man.
The boss ______ the worker how to do it, or the accident couldn’t have happened.
In the 1930s, the two countries were at ______ war, and his father was fighting at _____ front.
Don't leave for there until__________ to do so.