A parallel sentence mentions the same grammatical expression twice. I would go with "D". 'Passed' and 'Demonstrated' are both past tense words that express the same past tense grammatical idea.
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Answer:
Citing or documenting the sources used in your research serves three purposes: It gives proper credit to the authors of the words or ideas that you incorporated into your paper. It allows those who are reading your work to locate your sources, in order to learn more about the ideas that you include in your paper.