_______ difficult and challenging the task may be, we must finish it on time, because there is a possibility _______our work will go down in history.
The reason ____ she was absent is _____ he was badly ill yesterday.
It suddenly _____ to the detective that the professor was probably murdered by his own wife .
---What made her unhappy? ---_____.
The machine might break down someday in the future, ____, you can dial this number for help.
_________ returns first will win the big prize.