I think ________ lesson is the most difficult in this book．

1. A.
   five
2. B.
   fifth
3. C.
   the fifth
4. D.
   fifteen

I think ________ lesson is the most difficult in this book.

A. five

B. fifth

C. the fifth

D. fifteen

根据句子意思，用所给单词的正确形式填空。

I think the ________ lesson is the most difficult in this book. (four)