Classification is a useful ______ to the organization of knowledge in any field.
We must teach her a good lesson ___ she likes it or not.
We don't know ___ to pay a visit to the old town.
The question __ we will hold the sports-meet next Sunday will be decided very soon.
We were very much surprised at the way ___ she treated the old woman.
___ a terrible storm would take place in Hainan.