---You don’t often see them, do you? ---No, I see of them.
While building a tunnel through the mountains, ______.
Scientists are convinced ______ the positive effect of laughter ______ physical and mental health.
In American there are more households with pets than ______ with children.
They are said to have developed a new ______ to teaching, which is said to improve classroom teaching greatly.
The first one ______ will get a beautiful gift.