twice, the postman refused to deliver our letters unless we chained our dog.
We can never expect ____ bluer sky unless we create _____less polluted world.
Would you like tea or coffee?--_____, thank you. I’ve just had some water.
In that school, English is compulsory for all students, but French and Russian are ______.
Sometimes proper answers are not far to seek ______ food safety problems.
To get a better grade, you should ______ the notes again before the test.