______ I am concerned, the meeting held last week is a great success.

Perhaps the day will come ____ people will be able to breathe clean air in cities．

Hard _____ he tried, he failed to catch up with the first runner.

It is so difficult a question_______ none of the kids could work it out.

It will not be     he returns from abroad.

The professor, _____ some of his assistants, _____ to attend our discussion this afternoon.

It was in the Xichang Satellite Launch Center , ________ I had visited, ________ Chang’eⅠwas launched at about 6:05 pm October 24, 2007.

_____________ well prepared you are, you still need a lot of luck in mountain climbing.

The fleet made several expeditions _______ the exploration was stopped.

— Have you apologized to Mr. Jack?
— I’d shoot myself_______ I apologize to him.