—What ________ an important part in their lives?—Friends.
______ chemistry is a branch of chemistry.
More and more high-rise buildings have been built in big cities ______ space.
---We haven’t seen each other for ages. ---No, I don’t remember how many years ago _______ I last saw you.
We are still at the airport.If it weren’t for the delayed flight, we _______ the concert right now.
_____ his research paper, Professor Reagan experienced various hardships and loneliness for nearly half a year.