—Who______ it be that is knocking at the door? —It ______ be Father, but I’m not sure.
As there is less and less coal and oil, scientists are exploring new ways of making use of ______ energy, such as sunlight, wind and water for power and fuel.
In our class, when the bell rang and the teacher closed his book, it was a ______ for everyone to stand up.
Hiking by oneself can be fun and good for health. It may also be good for ______ building.
Some parents are just too protective. They want to ______ their kids from every kind of danger, real or imagined.
The loss has not yet been ______ accurately, but it is believed to be well beyond a hundred million dollars.