A warning signal suddenly appeared on the screen of my computer, ______ the computer was being attacked by a certain virus.
He went to the library ____ last night, but you broke your word.
We have to __________finding a solution.
His aunt’s letters ___of those beautiful days when they used to live together in his hometown.
When seeing a stone in front of the car, _____.
Smith wrote an article on ____ he failed to pass the exam.