A warning signal suddenly appeared on the screen of my computer, ______ the computer was being attacked by a certain virus.
This will be the second time that he ____________ China.
The policeman warned the driver ________ so carelessly.
Those ________ are not fit for the work should leave office at once.
When the expression first came into being, people refused to use it but _____ they began to accept it.
On hearing the sound, the birds flew away in every ________.