In the study of history, what is one major difference between a region and a period?

A. a region is not influenced by different historical turning points, but a period is.

B. A region can be studied over long spans of time, but a period applies to a specific time span.

C. A region can be studied only in modern era, but a period can be studied in any era.

D. A region can deal with many different themes, but a period deals with one specific theme