What does the term secondary source meaning

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Secondary source of information is one that was created later by someone who did not experience first-hand or participate in the events or conditions you’re researching. For the purposes of a historical research project, secondary sources are generally scholarly books and articles. Also included would be reference sources like encyclopedias.
Example: the book University of Illinois: The Shaping of the University could be used as a secondary source to learn about the UI around the turn of the twentieth century.
For secondary sources, often the best sources are those that have been published most recently. If you use a secondary source that was published decades ago, it's important to know what subsequent scholars have written on the topic and what criticism they made about the earlier work or the approach to the topic.

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-Kappifilms

Its is d. because i did it