Any content that you use under a Creative Commons license must be properly cited.
For example, a document or graphic image that you've found from an online source like Google or Creative Commons requires you to place a citation that displays the license information.
For examples of how your citations can be formed, check out "Best Practices for Attribution" from the Creative Commons Wiki. It has good reference material on concisely writing attributions for creative commons materials.
(Essentially, the rule of thumb is to make sure to include the Title, Author, Source and License.)
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