Not everyone has realized what ________important part the balance of ________nature plays in their life.
We must set about _____ our lesson for the mid-term examination.
I can't understand _____ at that poor child.
–Can you spare me some time? –I'm afraid not. I have lots of letters _____.
The patient was warned _____ oily food after the operation.
–I must apologize for _____ ahead of time. –That's all right.