The research is so designed that once _____ nothing can be done to change it.
As is known to all language learners, the newly learnt words will soon be forgotten unless______ used in everyday communication.
The last thirty years _________ the quick development in almost every field in China.
The dog, if trained _____, will be made a good watch-dog one day.
It is required that employees in the company have to have a good__________ of English.
We must keep on learning if we want to keep up with the ____ of the changing times.