We can’t figure out ______ quite a number of insects and birds are dying out.
It was when we were returning home ________ I realized what a good feeling it was to have helped someone in trouble.
______ scientists have learned a lot about the universe, there is much we still don't know.
If you come to visit China, you will ______ a culture of amazing depth and variety.
— Can you come to a party on Saturday, Peter? — Oh, ______I'm already going out, I'm afraid. A. what a pity! B. don't ask! B. C. how come? D. so what?
Much time _____ sitting at a desk, office workers are generally troubled by health problems.