It is required that employees in the company have to have a good__________ of English.
Finally, the thief handed over everything ____ he had stolen to the police.
The foreign guests, ____ were government officials, were warmly welcomed at the airport.
This is the very letter ____came last night.
I know only a little about this matter; you may ask ____ knows better than I.
This is the school ____ we visited three days ago.