Which of the following historians has organized her study of the past based on the category "period"?
a. a historian writes an extensive text on the important contributions of southeast asian countries
b. a historian studies each of the major world powers between the years 100 bce and 100 bc
c. a historian creates a graph of the economic strength of various empires throughout history.
d. a historian assesses a number of primary source documents to create a new secondary source.