Which materials are considered secondary data?
photos and letters given to you by another person
books and articles written by other authors about their studies
information that you have gathered and now have included in your results
responses from participants whom you both surveyed and interviewed

The correct answer choice from the options given, to answer the question: Which materials are considered secondary data? would be: Books and articles written by other authors about their studies.

Explanation:

The reason for this being the correct choice comes from learning that secondary data research is commonly described as using data and information provided by other authors, experts and researchers on a given topic, and that would answer the original research question. In this way, a researcher may gather already existing information on his topic of interest, learn about the background information and the knowledge already established, and work from there, instead of re-inventing something that was already there.