How to destory previous activity

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http://stackoverflow.com – So I have 2 activities lets say A and B. A navigates to B, I want the activity A to be killed or make it unusable/unseen when directed from B. so it should be like when I press the back button on B activity it should not open activity A instead it should go to the app tray. also activity A should comeback when I clear the application data thanks. (HowTos)