When we climbed up to the top of the mountain, we were all ___________.
The teacher set such a difficult problem ________none of us could solve.
If we want ______much cleaner world, we should reduce _______ emission of carbon dioxide.
This morning my mother told me _____ she would attend a wedding party _____ she wouldn’t come back home for lunch.
It is the test system, _____ the teachers, that is to blame for the heavy burden(负担) of middle school students nowadays.
Lucy was so excited _______she received an invitation from her friend to visit Beijing.