I think mathematics is very difficult ______.[ ]

Have you decided to go hiking this weekend— We have not been in ________ yet. So I’m not sure whether to go.

I believe this is the best movie of the year.— Well, .Anyway, the ending is OK.

— How was the party?— I don’t know. I ________ in without a ticket. [ ]

The number of the people who _______ cars ______ increasing in China.