—I have never been to the Great wall. —If you have an opportunity _______ there, you must take it.
I like English, ____ I am not good at it.
There is ____ office on _____first floor.
A: Is the pen on the desk? B: ____.
A: Is there a computer in your class? B: ____.
A: ____ boys and girls are there in your class? B: There’re21 boys and20 girls.