Please excuse me _______ your letter by mistake.
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.
If you are not happy with it, you should say it out. Don’t just ______ in silence.