He advised farmers to choose the best seed-heads ,__________ that had the best color .
| A．不填 | B．ones | C．this | D．the ones |
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 |
There are five pairs, but I'm at a loss which _____ to buy.
| A．to be chosen | B．to choose from | C．to choose | D．for choosing |