While Rose ______ for the lift, she is listening a recording about the Eiffel Tower.
--You don’t look ______ today. What’s wrong? --It’s the bad fever.
Sometimes parents find ______ hard to communicate with their children.
Nowadays scientists are working hard to find out ______ to do with air pollution.
Good teachers can usually give the students______ advice on their study.
Chinese people have been using paper for over______ years.