He was so deep in thought, thinking of the problem ________he knocked into the tree without noticing it.
Seeing the teacher raising her finger to her lips as __________ for silence, all the students sat down in a standard __________
Two big guns are fired, and it is the __________ for the celebrations to begin.
The red light is a __________ of stopping the cars.
They have read the paper many times, but it doesn't make any __________.
He's got himself into a dangerous situation __________ he is likely to lose control over the plane.