This dictionary is very useful. It has ___ many problems about words and idioms for me.

Thanks to scientific research, we now have a good understanding of ______ an earthquake is and why it happens.

They must have finished the work by the end of last month, ______?

You’ll have a difficult time knowing who your real friends are if you ______ rich in the future.

left the light on.——Oh, so I have. _____________to turn it off.