___ most of the earth’s surface is covered by water, fresh water is very rare and precious.
Natural disasters occur at ________ times, so it is always difficult to warn people in advance.
________ when your ticket has been checked will you be allowed to take the plane.
Sometimes we can see more than 60 students are ________ in a small school bus, which can easily cause traffic accidents.
— I heard the voices, but couldn’t ________ what they were saying. — Why not go over? It won’t do you any harm.
Who ________ a lady like Susan Boyle, once an ordinary woman in Scotland, would achieve overnight success at Britain’s Got Talent Show?