How to force quit an OS X app that isn't responding

[iMore.com]

While OS X may have the reputation of being a solid operating system, it isn't immune from occasional crashes and app freezes. If you've owned a Mac for a long period of time, you've most likely come across the spinning beach ball of death a time or two. Here's how to figure out which app is causing the problem, if you don't know already, and how to force quit the app:

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