—Who______ it be that is knocking at the door? —It ______ be Father, but I’m not sure.
—— Oh, it’s raining hard outside. Mum, do you think the milkman will come?—— Of course. He will come on time _____it rains.
Though the book is a little expensive, it is _____.
It is required that students _____mobile phones at school, so seldom _____them using one.
——Tom is never late for work. Why is he absent today?—— Something _____to him.
—— Could you tell me his telephone number?—— _____the phone book and find it out yourself.