_______, the experiment will be successful.
| A．If carefully doing | B．If it done carefully |
| C．If carefully done | D．If doing carefully |
At school, some students are active ______ some are shy, yet they can be good friends with one another.
| A．while | B．although | C．so | D．as |
（2012辽宁卷）Not until he retired from teaching three years ago having a holiday abroad.
| A． he had considered | B． had he considered |
| C． he considered | D． did he consider |
It was 80 years before Christopher Columbus crossed the Atlantic______ Zheng sailed to East Africa
| A．when | B．that | C．after | D．since |
The headmaster will not permit the change in the course, nor______ it a thought.
| A．does he even given | B．he even gives | C．whether | D．he will even given |
It was not until I came here ____ I realized this place was famous, for not only its beauty but also its weather.
| A．who | B．that | C．where | D．before |