Only when_______ 30 years old ______ to learn English.
"Why did you go to the wrong class, Mr. Wang " "Well, I forget _______ I was supposed to go to."
I don't doubt ________ he'll come.
Why the explosion occurred was ____the laboratory attendant had been careless.
They came to the conclusion ________ by a computer.
________ is no reason for dismissing her.