How is organizing history by period different from organizing history by theme?

A historian who organizes the past by region focuses on a specific start and end date, whereas a historian who organizes the past by period focuses on the most influential individuals" would be the best option from the list, but in reality historians who focus on a specific period take into account many different factors of life during that period.

the use of email to replace slower surface mail.

just finished this it is b. centrifugal force