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CC0 - Public domain

CC0 (occasionally written as CC Zero) is a public domain dedication that allows copyright holders to place works in the public domain to the extent legally possible, worldwide. When a work is in the public domain, it is free for use by anyone for any purpose without restriction under copyright law. Public domain is the purest form of open/free, since no one owns or controls the material in any way. (Read more about this license)
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CC BY - Creative Commons Attribution

This license lets others distribute, remix, tweak, and build upon your work, even commercially, as long as they credit you for the original creation. This is the most accommodating of licenses offered. (Read more about this license)

CC BY-SA - Creative Commons Attribution-Share Alike

This license lets others remix, tweak, and build upon your work even for commercial purposes, as long as they credit you and license their new creations under the identical terms. This license is often compared to “copyleft” free and open source software licenses. 
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