Historians identify certain historical events as turning points in order to:
a. emphasize that certain themes are insignificant when focusing on
a single time period.
o
b. compare similar events that took place in different regions of the
world.
o
c. study significant past events without needing to find primary
source evidence.
o
d. determine which events caused the most change between
historical periods.

D - Determine which events caused the most changes between

historical periods.

Explanation:

Historians take significant events as milestones that have had a high impact in the world and have determined the historical course to a greater or lesser extent.  

For example, the beginning of the Middle Ages, or the end of the ancient period, take the fall of the Western Roman Empire 476th year and lasts until the discovery of America in 1492. These two major historical events brought about tremendous changes in the past of human society and markedly determined the future history course.