根据句意，用所给单词的适当形式填空。

小题1:We all know ______________ (pollute) water does harm to our health.

小题2:I have read five                  (twelve) of this book.

小题3:If you are in a blue mood, you can do your hobbies to cheer                ( you) up, boys.

小题4:To tell you the                 (true), I felt sad while you were away. 

小题5:The children heard the                 (frighten) shout and looked at each other in fear.