A: Would you like to go to the concert after supper? B: I have to look after the twins at home tonight.
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.
In my opinion, all Mr. White ___ good to his students in his class at present. He is very strict in their study.