根据句意和首字母提示完成单词。   

1．The world’s d___ salt lake is the Caspian Sea．

2．The ______ of the world is about 6，000，000，000．

3．Are there any ________ between this book and that one?

4．My father has been much better since he g___ up smoking．

5．It's hard for us to make a d_____ ．