______ it______ for your help,I couldn't have made any progress.
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.
The most important thing _______ we should pay attention to is the first thing ________ I have said.
_____, the great mother, in order to save her sick child, kept waiting for four hours in the heavy snow.
Was it in 2002 __________ he was still at middle school __________ the boy became expert at computer?