How is organizing the past by theme different from organizing the past by region?

A. historian who organizes the past by theme studies a big idea
that occurs across history, whereas a historian who organizes by
region focuses on specific start and end dates.
B. A historian who organizes the past by theme studies a big idea
that occurs across human history, whereas a historian who
organizes the past by region focuses on a specific area of the
world.
C. A historian who organizes the past by theme focuses on a specific
individual, whereas a historian who organizes the past by region
focuses on a big idea from history.
D. A historian who organizes the past by theme studies a specific
time period, whereas a historian who organizes the past by region
studies a specific individual from human history,