I like getting up very early in summer. The morning air is so good____.
| A．to breathe | B．to be breathed |
| C．breathing | D．being breathed |
It was when we were returning home ________ I realized what a good feeling it was to have helped someone in trouble.
| A．which | B．that |
| C．where | D．how |
______ scientists have learned a lot about the universe, there is much we still don't know.
| A．Once | B．Since | C．Though | D．Unless |
If you come to visit China, you will ______ a culture of amazing depth and variety.
| A．develop | B．create |
| C．substitute | D．experience |
— 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.
| A．being spent | B．having spent |
| C．spent | D．spending |