Our maths teacher set so difficult an examination problem ______ none of us worked out.

The police have ____ power to deal withsuch matters by ____ law．

The president stood by a window inside the room, ____, looking over the square.

There’s no bus now, and we can’t get home __ by taxi.[ ]

Have you finished the book?— No, I’ve read up to ________ the children discover the secret cave[ ]