31. Look at the notice. You are not allowed _______ the gymnasium without permission.
_______ the teacher’s instruction in the lab_____ important.
E-mail, as well as telephones, ______ an important part in daily communication.
Professor Wang has written some short stories, but he is______ known for his plays.
This is the very information _______ we can learn what is selling best in the market. .
All _____ can be done has already been done.