Students often ______ a lot of difficulties when they communicate with foreigners for the first time.

I thought my eyes must be playing tricks _______ me

It is written clearly in the requirementsthat all passengers ______ check onto board 30 minutes before the plane takesoff

The environmental problems in China may worsen if__________ unsolved.

What’s DIY exactly, Suzy—It_____ “do-it-yourself”, you do something yourself instead of _____ someone to do it.