根据句意，写出括号内所给单词的适当形式。

1.Many of my friends were invited to the party last Sunday because of my _________ ( twenty ) birthday. 

2.Boys, don’t touch the machines, or you may hurt _________ ( you ). 

3.The International Union Against Cancer says about seven hundred million children _________ ( breath ) smoke-filled air. 

4.I can’t understand his             (silent) on such an important matter.