What is the author’s likely purpose for including direct quotations from Sigmund Freud’s works throughout the article?

A. To argue that Freud’s theories are undisputed and his methods are considered the way to behavioral therapy

B. To reveal that these ideas were only held by Freud himself

C. To support the idea of that Freud is the only psychologists who understands the complexity of the unconscious mind

D. To lend authority to the central ideas by citing from a reputable primary source