When _______ there, the postman found the door locked and then stopped _______ the letter in the mailbox on the wall.
When he knocked at the door, my mother was busy ____ supper.
He waited until it was dark ____ nobody could see him clearly.
Now there are many _____ in our company.
We should think of ways to stop the waste from the factory _____ our living environment.
As far as I know, this man ____ risk anything than lose the ring for his wife.