A signal phrase is a phrase or clause that introduces a quotation, summary or paraphrases something. Signal phrases always requires a verb and it also may include the name of the person or the source where the information can be found. Option C, <em>as described in</em>, introduces information and after these words (in the passage) it is possible to find the authors, Boyd and Ellison, so it is the correct answer.
If you don’t learn history you are condemned to repeat it. Children sometimes don’t learn and repeat bad things so not learning history is kinda like that.
