Whatever work you are doing, you should concentrate your attention ______it

Isn’t it Ann’s husband over there?—- No, it ______ be him—I’m sure he doesn’t wear glasses.

Do you think the thief entered through the window?—No, if he had, I don’t believe, _______ broken the living-room’s window.

I will tell him the good news he __________ home from work.

Whose pencil box is this—It _____ be Tom’s. It has his name on it.