How does the professor explain the geographical origins of racial groups?
A    By describing each racial group and their geographical origins in the 19th
 century.
B    By stating two different views on the geographical origins of racial groups in the 19th
 century.
C    By  comparing  the 19th
  century view on  the geographical origins of  racial  groups with  that of today.
D    By giving an example of how to divide human racial groups geographically.
(C)