Mary _________ as a volunteer teacher for 10 years.
I didn’t see you when I went to your office yesterday afternoon. Where were you?—I ____ a meeting in the meeting room.
The problem is very difficult. I’m afraid you can’t work it out now.—I fail this time, I will try it again.
It’s dangerous to swim here. Look at the sign!—Oh, I _______ it.
Only when your identity has been checked, ___________.