1. (2015八上·三门期中) 用所给单词的适当形式填空。

common,   reason,   serious,   free,   help

1. （1） One of the main is that this book is interesting, so it is very popular.
3. （2） He can't go with us. He has to his parents at home.
5. （3） I'm this evening. I want to go to the movies.
7. （4） English is important, you should take it .
9. （5） These books have nothing in .