Reading a Secondary Source on the Topic of Research About

Reading a secondary source on the topic of research about class size is of particular benefit to a researcher planning to carry out a study involving this variable because

A) it provides the direct perspective of the individuals who conducted the research.
B) it combines knowledge from many primary sources into a single publication.
C) it provides an exhaustive list of preliminary sources relevant to the topic of class size.
D) it eliminates the need to do one's own synthesis of the primary source literature.