Idioms and vocabulary

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1) Air pollution irritates our respiratory systems or Air pollution irritates our respiratory system ? -- Both are OK.  I'd use the former.

2) Could you tell me the idiom of which the meaning is: You are angry about something and you blame it on something else?-- I can think of no single idiom for both ideas.

3) The average number of the population of a nation (number) divided by the area (km), how do we call the result? -- Population density.

4) What is the Aussie way of saying good-bye ? --  Dunno, mate

5) Do we ever add s to vocabulary, ie vocabularies? -- Only rarely