The correct answers are B and D.
In order to get accurate and correct information when doing research, it is very important to resort to credible sources.
A study done by researchers would be a good source of information because, in order to be published on the University website, the study has to be reviewed and approved by the University faculty members.
A book written by a scholar on the topic would probably be the best source available, because it contains reliable data provided by an important figure. Since the writter of the book is a professional on the topic, we can be sure that the information provided is correct.