—Who______ it be that is knocking at the door? —It ______ be Father, but I’m not sure.
--Do you know anyone in Paris? --No, I’ll make friends once______ .
The research is so designed that once ______ nothing can be done to change it.
He is always______fault with other people though he doesn’t do his own work properly.
Meeting my uncle after all these years was an unforgettable moment, ____ I will always treasure.
Hardly ______ when the bus suddenly pulled away.