Give them a hand this time and I am sure they will offer theirs ______ in the future.
—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.