We haven’t settled the question of __________ it is necessary for him to study abroad.
Eye doctors recommend that a child’s first eye exam ____at the age of six months old.
______how others react to the book you have just read creates an added pleasure.
If we leave right away, ____we’ll arrive on time.
The children, ____ had played the whole day long, were worn out.
As the world’s population continues to grow, the ___of food becomes more and more of a concern.