You can complain, but I doubt _____ it’ll make any difference.
An awful accident, however, _____occur the other day.
It was probably at that moment _____ I understood what kindness really was.
—What was it that caused the party to be put off?—________ the invitations.
Who ________ that played a trick on the new English teacher?
—Where did you pick up the stamp?—It was at the post office ________ we worked in the summer vacation.