---I’m afraid I ______ return the book to you before Friday. --- Take your time.
—I usually go there by train.—Why not __________ by boat for a change?
Having been ill in bed for nearly a month, he had problems __________ the exam.
It was so dark that he __________.
The Wangs are considering __________ to Huangshan for a trip at five tomorrow morning.
The rule is true __________ all cases.