In view of the worsening environment, the local government is seeking advice from Professor Smith, who is an expert ______ it.
We ______ to clean the blackboard every day.
The old man is ______ under the tree and ______ a paper.
-Can you stand ______ in that way?- Absolutely not.
Don't forget ______ the light when you leave the office.
You'd better not ______ your coat, or you'll get cold.