27. _____ the teacher came in, everyone was quiet.
We found ________ necessary to protect the environment.
-Which would you like, rice or noodles? --_______is OK. I'm hungry.
I don't think it very expensive to buy a family computer here. -- Really? I'll buy _______ next week.
Lucy knew nothing about it ________ her sister told her.
He walked ________ fast for us ________catch up with.