They often change the kind of crop in each field ____ few years, for example, growing corn or wheat and then the next year peas or soybeans.
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.