The old lady was nervous, _____ she wasn’t used to strangers calling late at night.
ack went to college last year, _______ to return home until he achieves his goal.
_______ in the queue for half an hour, Tom suddenly realized that he had left his wallet at home.
_______ to sunlight for too much time will do harm to one’s skin.
The bell _______ the end of the period rang, _______ our heated discussion.
The research is so designed that, once _______, nothing can be done to change it.