--- Is Mr. Smith in the office? --- Yes,_____ he is in charge of the office,he must be there.
He was born ____ a very cold night _____ December.
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.