—Where did you get this book?—I picked it up from a second­hand bookshop if you ________ know.

One of our rules is that every student ______wear school uniform while at school.

We can hardly believe such a young boy ______ travel around the world all by himself.

_____ anyone phone during the board meeting, please tell them I am busy.

—I think the man over there must be Bob. -- It_______ be him. He has______ to Australia.