The plane will take off _______ three hours. I must get to the airport right now. [ ]

I don’t think ______ 18-year-old boy is serious enough to drive.

I found ___________ hard to remember all the twelve star signs because they are difficult to pronounce(发音) and spell.

is the ticket of the museum for children? — Well, it’s free for everyone.

TheBellToweris_____themiddleofXi’an.[ ]