I'm very grateful for your help and hope that someday I may be able to do something for you________.
| A．in exchange |
| B．in turn |
| C．in return |
| D．in particular |
In that month, he earned as much as, if ____ than, $40 000.
| A．no more | B．not more | C．no much | D．not much |
The research is so designed that once _____ nothing can be done to change it.
| A．begins | B．having begun | C．beginning | D．begun |
If _____ carefully, the experiment will be successful.
| A．do | B．does | C．done | D．doing |
He is a man of few words, and seldom speaks until _____ to.
| A．spoken | B．speaking | C．speak | D．be spoken |
They kept trying _____ they must have known it was hopeless.
| A．if | B．because | C．when | D．where |