_________ life will continue on the earth for millions of years to come will depend on __________ this problem can be solved.
Only after Mary read her composition the second time _________ the spelling mistake.
______ being fun and good exercise ,swimming is a very useful skill.
Come and see me whenever ____.
It is strange that he _________ in such an important examination .
After ten hours’ operation, the doctors managed to _____the one-year-old twins at the head.