Why is it useful for historians to base certain Studies on the analysis of historical periods?<br />
a. the major historical themes apply only to certain civilizations during certain periods<br />
b. the ideas that characterize a particular period are unaffected by ideas from other periods<br />
c. similar events and trend often take place on different places during the same periods<br />
d. experience has confirmed the analyzing periods is the most effective approach to history
